Rocky desertification is serious in karst areas in southwest China, with low vegetation coverage and poor soil. As an important ecological management measure, vegetation restoration plays an extremely important role in improving the availability of soil nutrients, increasing biodiversity, and improving the ecological environment. In this study, four types of site (the pure plantation of Dalbergia odorifera; the pure plantation of Acricarpus fraxinifolius; the mixed plantation of D. odorifera and A. fraxinifolius; the adjacent natural weed field as the control) in Mashan, Guangxi were selected as the research objects after 8 years of vegetation restoration. The effects of introducing N₂-fixing tree species (D. odorifera and A. fraxinifolius) and their planting patterns on the soil phosphorus transformation and availability in rocky desertification mountainous regions were discussed. The results showed that:(1) the contents of soil organic matter (SOM), nitrate nitrogen (NO₃^--N), available phosphorus (AP), soil microbial biomass carbon (MBC), soil microbial biomass nitrogen (MBN), and soil microbial biomass phosphorus (MBP) in the mixed plantation of D. odorifera and A. fraxinifolius were significantly higher than those of the control (P<0.05). The total phosphorus (TP) of the mixed plantation of D. odorifera and A. fraxinifolius, the pure plantation of A. fraxinifolius and the pure plantation of D. odorifera could be easily transformed into AP than control, among which the mixed plantation of D. odorifera and A. fraxinifolius had the best effect. (2) The results of correlation analysis showed that NH₄⁺-N, NO₃^--N, and MBN had significant linear positive correlation with AP respectively (P<0.05), while MBN and MBP had extremely significant linear positive correlation (P<0.01). (3) After introducing D. odorifera and A. fraxinifolius, the soil inorganic phosphorus fractions in all sites changed, and the contents of various inorganic phosphorus in the mixed plantation of D. odorifera and A. fraxinifolius and the pure plantation of A. fraxinifolius increased significantly (P<0.05). (4) Finally, the redundancy analysis (RDA) indicated that MBP, SOM, and MBN would be the main factors affecting the changes of soil inorganic phosphorus fractions (P<0.05). Overall, D. odorifera and A. fraxinifolius could improve soil nutrient and phosphorus availability in rocky desertification mountain regions to a certain extent, among which the mixed planting pattern (the mixed plantation of D. odorifera and A. fraxinifolius) had the most significant effect. The purpose of the study could provide theoretical basis and practical support for the application of N₂-fixing tree species in the ecological restoration and reconstruction of rocky desertification.