Abstract: Phosphorus has been one of the most limiting factors in the nonrenewable natural resource to the food security in modern agriculture. The availability of phosphorus is reduced in coastal saline soil, due to the influence of pH value and salt. Biochar can be expected to serve as a coastal saline soil amendment for better soil properties. However, a new slow-release fertilizer, ammonium polyphosphate cannot be easy to be fixed in the soil. Therefore, this study aims to clarify the effects of ammonium polyphosphate combined with biochar and straw on the transformation characteristics of available phosphorus and inorganic phosphorus in the coastal saline soil. An indoor culture experiment was conducted. Five treatments were set as the blank (CK), ammonium polyphosphate (OA), ammonium polyphosphate with biochar (AB), ammonium polyphosphate with straw (AS), and ammonium polyphosphate with biochar and straw (ABS). The improved Hedley phosphorus grading method with a 100-mesh sieve was used to grind the soil in the different culture periods. A total of six inorganic phosphorus and forms content were obtained after continuous extraction, such as resin-exchanged phosphorus (Resin-P), NaHCO3-extracted phosphorus (NaHCO3-P), NaOH-extracted phosphorus (NaOH-P), 1mol/L HCl-extracted phosphorus (D. HCl-P), concentrated hydrochloric acid-extracted phosphorus (C. HCl-P), and residual phosphorus (Residue-P). The correlation was analyzed between the pH, available phosphorus, and inorganic phosphorus forms. The results showed that: 1) The available phosphorus content of ammonium polyphosphate treatment significantly increased by 2.46-4.02 times (P<0.05), compared with the CK, during the incubation period. The ABS treatment significantly increased by 15.01% and 19.20% at the end of culture, compared with the AB and AS (P<0.05). 2) There were different trends in the content of Resin-P under different treatments with the prolongation of culture time. The OA and AB treatments showed an increasing trend, whereas, the AS and ABS treatments showed a decreasing trend. At the end of the culture, the content of Resin-P in the AS and ABS treatments was significantly lower than that in the OA treatment by 39.98% and 31.06%, respectively. The content of NaHCO3-P increased first and then decreased with time, reaching the maximum on the 28th day of culture. The maximum was found in the ABS treatment. The content of NaOH-P and D. HCl-P increased with the prolongation of culture time. The content of NaOH-P in the ABS treatment was significantly higher than that in the OA by 54.07% (P<0.05) at the end of the culture. The contents of C. HCl-P and Residue-P decreased as a whole. The contents of Residue-P in the AB and ABS treatments significantly increased by 34.01% and 26.12%, compared with the OA (P<0.05). 3) Phosphorus in the coastal saline soil mainly existed in the form of HCl-P and Residue-P, accounting for 88.60%-92.20%. Correlation analysis showed that the correlation coefficients between different phosphorus forms and soil-available phosphorus were in the order of D. HCl-P> Resin-P> NaOH-P> NaHCO3-P> Residue-P>C. HCl-P. Except for the C. HCl-P, there was a significant positive correlation between other inorganic phosphorus forms. In summary, the ammonium polyphosphate combined with the biochar and straw (ABS) is superior to improve the effectiveness of phosphorus literacy in the coastal saline soil, while, the Resin-P and D. HCl-P contributed the most to the available phosphorus. This finding can provide an important theoretical reference for the phosphorus availability and utilization rate in coastal saline soil.