Abstract: Low available soil phosphorus (P) limits alfalfa (Medicago sativa L.) growth in the semi-arid Loess Plateau. Understanding the effects of continuous film mulch and P application on soil P fractions in alfalfa grasslands is crucial for optimizing P management and improving productivity. Two mulch treatments (no film mulch and film mulch) and four P fertilizer regimes 0, 9.7, 19.2, and 28.8 kg(P)∙hm⁻² were employed to determine their effects on soil P fractions and their determinants in alfalfa grassland over different treatment durations (years 3 and 9). The results revealed that film mulching significantly increased P uptake by alfalfa shoots by 52.4%. Furthermore, mulching decreased soil Olsen P, high-activity NaHCO₃-P_o and NaHCO₃-P_i, and medium-activity NaOH-P_i contents by 15.5%, 20.2% and 10.2%, and 27.9% in year 3, respectively. In year 9, film mulch markedly increased soil NaHCO₃-P_o and NaHCO₃-P_i contents by 37.3% and 24.3%, respectively, but had no significant effect on alfalfa shoot P uptake and soil Olsen P and NaOH-P_i contents. Across both experimental years, film mulch reduced the content of diluted low-activity HCl-P_i, while significantly increasing soil NaOH-P_o, concentrated HCl-P_o, and residual-P contents. However, it had no significant effect on concentrated HCl-P_i and total P contents. Comparatively, P application significantly increased alfalfa shoot P uptake, soil total P, Olsen P, and all P fractions in years 3 and 9 compared with those of no P application, except for soil residual-P content in year 3. Redundancy analysis indicated that shoot P uptake, soil pH, and alkaline phosphatase activity were the main factors affecting changes in soil P fractions. Film mulch significantly increased soil alkaline phosphatase activity in both experimental years and decreased soil pH in year 9 compared with that of the mulch treatment. P application significantly decreased soil pH in years 3 and 9 and reduced soil alkaline phosphatase activity in year 9 compared with that of no P application. In conclusion, film mulch promotes the transformation of low-activity P fractions into medium- and high-activity P fractions by reducing soil pH and enhancing phosphatase activity. Therefore, the combination of film mulch and P application can improve the availability and supply potential of soil P in alfalfa grasslands, and these benefits increase over time.