[Objective] To investigate the effects of pea-oat mixed cropping at different ratios on soil nutrients, enzyme activities, and carbon-fixing microbial communities in the alpine region of Qinghai Province. [Methods] This study designed five treatments: monoculture of oat (T0), monoculture of pea (Q0), and pea-oat mixed cropping at three ratios (QT1, QT2, and QT3). Soil enzyme activities and physicochemical indicators were measured, and the community structure, diversity, and relationships with environmental factors of carbon-fixing bacteria were analyzed based on high-throughput sequencing data of cbbL. [Results] Compared with monoculture, mixed cropping significantly enhanced soil enzyme activities and nutrient content, with QT3 (the pea:oat ratio of 2:1) showing the most pronounced effects. Under QT3, the activities of sucrase, cellulase, protease, and other enzymes were the highest, and the content of soil organic carbon, total nitrogen, ammonium nitrogen, and other nutrients was also significantly increased. Mixed cropping optimized the community structure of soil carbon-fixing bacteria. Specifically, QT3 significantly increased the alpha diversity (with the highest Chao1 and Shannon indices) and enriched functional groups such as Cyanobacteria. Environmental factor analysis indicated that soil ammonium nitrogen, total nitrogen, and organic carbon were the main drivers of changes in the carbon-fixing bacterial community. Furthermore, microbial co-occurrence network analysis revealed that mixed cropping, especially QT3, significantly enhanced network complexity, connectivity, and modularity, which indicated higher structural stability and functional collaboration potential of the microbial community. [Conclusion] Pea-oat mixed cropping, particularly at a ratio of 2:1, significantly improved soil ecological functions in alpine regions by enhancing soil enzyme activities, increasing nutrient availability, and optimizing the structure and functional network of carbon-fixing microbial communities. This provides an effective approach for enhancing soil carbon sequestration potential and promoting sustainable agricultural development.