Abstract: This study aims to figure out the optimum stocking density of weaned lambs in northern China. A series of experiments were carried out to investigate the impact of different stocking densities on the growth performance, serum indexes of physiology and biochemistry, as well as the rumen microbial fermentation of weaned lambs. The experimental animals were selected male Australian white lambs with an initial weight of (14.78 ± 2.15) kg. Seventy-two lambs were then divided into three groups, according to the different stocking densities: 0.875, 0.625, 0.375 m²/lamb. The lambs of each group with three replicates were fed a mixture diet of corn stalk, corn, concentrate, premix. The suitable stocking density was fully investigated to evaluate, according to the production performance, rumen microbial fermentation, physiological, and biochemical indicators. The results indicated that the body weight gain, average daily gain, and average daily feed intakes of weaned lambs were positively related to the increasing stocking density, while the feed/gain was negatively related (P < 0.05). The lowest feed/gain was observed in the group with 0.625 m²/lamb. Furthermore, the apparent digestibility of dry matter and crude protein showed a quadratic increase with the increasing stocking density (P < 0.01) after chemical analysis, whereas, there was no variation in the digestibility of rest nutrients. In serum physiological and biochemical parameters, there was a linear increase with the high density of stocking in the total cholesterol and triglyceride concentrations of the serum. While the increasing stocking density significantly increased the immunoglobulin G, immunoglobulin A, interleukin-6, interleukin-10, tumor necrosis factor-α (P < 0.05). In addition, there were significantly increasing levels of superoxide dismutase, catalase, and glutathione peroxidase in the serum, particularly with the increasing stocking density (P < 0.05). In terms of rumen fermentation and microbial composition, the rising stocking density resulted in a linear increase in the total volatile fatty acids and propionate concentration in the rumen of early-weaned lambs, except for a notable decrease in the acetate/propionate ratio (P < 0.05). According to 16S rRNA sequencing, the stocking density of weaned lambs shared a significantly positive influence on the relative abundance of unclassified_c_Clostridia. The relative abundance of Olsenella was negatively affected by the increasing stocking density (P < 0.05). Correlation analysis revealed that there was a positive relationship between unclassified_c_Clostridia and body gain, average daily gain, and average daily feed intake, while a negative relationship was observed with feed/body (P < 0.05). However, there was a negative correlation between Olsenella, body gain, average daily gain, and average daily feed intake. However there was a positive correlation with the feed/body (P < 0.05). In conclusion, the increasing stocking density positively resulted in the growth performance, the apparent digestibility, and rumen propionate fermentation of weaned lambs. Conversely, the stocking density of 0.375 m²/lamb presented adverse effects on immunity, oxidative stress state, and rumen microbial fermentation. Thus, the stocking density of 0.625 m²/lamb was recommended for the weaned lambs. The finding can also provide the engineering technical parameters in standardized facilities for the weaned lambs.