Objective To investigate the physicochemical properties of aluminum phosphate adjuvants subjected to different numbers of autoclaving cycles and to evaluate the effect of aluminum phosphate adjuvant with varying autoclaving cycles on immunogenicity of type 6A pneumococcal polysaccharide-tetanus toxoid (TT) conjugate (6A-TT).Methods Self-prepared aluminum phosphate adjuvants were autoclaved 1, 2, and 3 times, respectively. The particle size and distribution, pH, isoelectric point, free phosphate content, phosphorus-to-aluminum molar ratio, and protein adsorption rate were examined. Additionally, the effect of aluminum phosphate adjuvants treated with different autoclaving cycles, when combined with 6A-TT, on the specific antibody titers in mice was assessed.Results After autoclaving 1, 2, and 3 times, the 50% particle distribution diameters (Dv50s) of aluminum phosphate adjuvant were 5.04, 5.22, and 5.12 μm, while Dv90s were 9.20, 9.87, and 10.60 μm, respectively. The pH values were 4.6, 4.3, and 4.3, while the isoelectric points were 5.48, 5.36, and 5.29, respectively. The free phosphate content were 4.12%, 4.79%, and 4.86%, and the phosphorus-to-aluminum molar ratios were 1.012, 0.972, and 0.944, respectively. The adsorption rates of aluminum adjuvant to TT were 92.7%, 87.8%, and 85.7%, and to 6A-TT were 93.8%, 91.6%, and 90.1%, respectively. The aluminum phosphate adjuvant, after autoclaving 1, 2, and 3 times, was able to adsorb 6A-TT and induce antibody production in mice, with geometric mean titers of 1 213, 985, and 1 056, respectively.Conclusions Increasing the number of autoclaving cycles leads to decrease in the isoelectric point and pH of the aluminum phosphate adjuvant, increase in free phosphate content, and reduction in the phosphorus-to-aluminum molar ratios and protein adsorption rate. It may also have an effect on the immunogenicity of 6A-TT pneumococcal polysaccharide antigen in mice.