In order to reveal the effect of phosphorus fertilization on soil phosphorus fractions and availability in alpine grassland, 0, 10, 20, 30, 40, 50 and 60 g/m² of phosphorus (P₂O₅, 16%) fertilization gradients were set with random block design in northwestern Sichuan. The content of soil phosphorus fractions and available phosphorus (Olsen-P) were estimated. The results showed that: (1) Soil total phosphorus (TP) increased firstly and then varied slightly but Olsen-P decreased significantly with increasing phosphorus fertilization doses. NaOH-Po and Residual-P were the main fractions accumulated in 50 and 60 g/m² phosphorus fertilization treatments in alpine grassland, and their contents were significantly higher than that in non-fertilization treatment; (2) Soil Resin-Pi, NaHCO₃-Pi, NaHCO₃-Po and NaOH-Pi increased firstly and then decreased with increasing phosphorus fertilization doses, and they peaked at 21.54, 22.94, 65.86 and 64.48 mg/kg², respectively, under 30 g/m² phosphorus fertilization treatment in the topsoil. HCl-Pi decreased with phosphorus fertilization doses; (3) Random Forest regression and Path analysis showed that NaOH-Po and Residual-P were the sinks of Olsen-P transformation. NaOH-Po negatively correlated with soil pH and Residual-P positively correlated with Fe, Al content and microbial biomass phosphorus (MBP). Thus, when we applied phosphorus fertilization to alpine grassland, Olsen-P content could be enhanced by reducing NaOH-Po and Residual-P accumulation through regulating soil pH, Fe, Al content and MBP. The results are expected to provide clues for future fertilization management in alpine grassland of the northwestern Sichuan.