Abstract: The annual total amount of livestock and poultry manure produced in China can reach approximately 3.05 billion tons. Among them, pigs, as one of the main types of livestock raised, have a large number of animals in stock and a wide range of breeding areas. The amount of pig manure accounts for a relatively high proportion of the total manure produced by livestock and poultry in China. Therefore, the treatment of pig manure has become a key focus in the pollution control and prevention of breeding industry in China. Antibiotics are widely used in livestock and poultry breeding industry for therapeutic and disease prevention. Veterinary antibiotics administered to animals in the breeding industry are not completely metabolized by the animals. Approximately 20% to 97% of these antibiotics are excreted in their active forms via manure, leading to antibiotic residues in the environment and posing threats to ecosystems and human health. Sulfamethoxazole (SMX) is a widely used veterinary antibiotic in the livestock and poultry breeding industry. Due to its stable chemical properties, it can persist in the environment long-term after being discharged via livestock manure. Furthermore, some degradation metabolites of SMX in the environment exhibit significantly higher toxicity than the SMX itself. Among them, the toxicity of 4-hydroxyl-N-(5-methyl-1,2-oxazole-3-yl)benzene-1-sulfonamide (4-OH-SMX) and 4-nitro-sulfamethoxazole (4-NO2-SMX) is 5.7 times and 21.4 times that of SMX respectively. Previous researches primarily focused on the residual levels of SMX in the environment, with less attention given to its higher toxic metabolites. China is the world's largest pork producer, facultative anaerobic composting is a major method for the harmless treatment of pig manure. This research comprehensively investigates the degradation dynamics of SMX and its seven important metabolites during the facultative anaerobic composting process of pig manure. The study systematically evaluated the influence of varying ambient temperatures (35 ℃, 25 ℃, 15 ℃) and distinct initial moisture contents of the raw manure substrate (70%, 75%, 80%) on these processes over a defined experimental period. Following a 90-day facultative anaerobic composting process across all tested conditions, the results demonstrated a substantial degradation efficiency for SMX, ranging from 64.82% to 78.04%. Notably, the most effective SMX removal was achieved under the combined conditions of 35 ℃ composting temperature and 80% moisture content, highlighting the synergistic enhancement of SMX degradation facilitated by elevated temperatures coupled with higher moisture levels. Analysis of metabolic transformation pathways revealed the detection of six distinct SMX metabolites throughout the facultative anaerobic composting precesses. Among them, 3-amino-5-methylisoxazole emerged as the predominant transformation product, reaching a peak detected concentration of 4.13 mg/kg. Its formation pathway was predicted by detailed structural characterization, which involving the enzymatic cleavage of the S-N bond within the parent SMX molecule, likely mediated by specific bacterial metabolic activities inherent to the composting microbiome. Critically, the study also identified the presence of 2 highly toxic metabolites, 4-OH-SMX and 4-NO₂-SMX after composting. The maximum detected concentrations of 4-OH-SMX and 4-NO₂-SMX were 0.12 mg/kg and 0.03 mg/kg, respectively. An important observation was that the generation of 4-NO₂-SMX exhibited a temperature dependency, with its highest formation concentration occurring specifically at 25 ℃. Comparatively, the production of 4-OH-SMX behaved lower sensitivity to fluctuations in either composting temperature or the initial moisture content of the manure. This key finding indicates that the inherent biochemical pathways leading to 4-OH-SMX formation are remarkably persistent under standard composting conditions and cannot be effectively suppressed or avoided solely through temperature or moisture adjustments within the ranges tested. Consequently, the unavoidable presence of 4-OH-SMX necessitates the implementation of additional, targeted post-composting treatment strategies to effectively mitigate this residual toxicity and significantly enhance the overall environmental safety and suitability of the final compost product for subsequent agricultural utilization. Therefore, it is suggested in this study that turning or aerating the compost piles during the facultative anaerobic process, coupled with maintaining the temperature above 30 ℃, could probably help reduce the production of these highly toxic transformation products. In conclusion, the empirical findings and mechanistic insights proposed in this study offer substantial and actionable technical support for optimizing pig manure treatment protocols and provide a scientific basis for designing manure management strategies aimed at maximizing the degradation efficiency of residual SMX antibiotics while simultaneously minimizing the accumulation and potential environmental risks posed by its highly toxic transformation products, ultimately promoting safer resource utilization of pig manure.