The biological nitrogen input is essential for nutrient enrichment in natural grassland ecosystems. This process is carried out by diazotrophic microbes. However, the patterns and assembly processes of diazotrophic communities in grassland soil are not fully understood, especially in alpine meadows of high-altitude Qinghai-Tibet Plateau (QTP). In this study, the diversity patterns and community assembly processes of soil diazotrophic communities were investigated in QTP alpine meadows. The results showed that different diazotrophic taxa responded differentially to the increase of soil pH, and α-diversity increased linearly with pH values. Diazotrophic community turnover was enhanced by the increased variation in soil pH, moreover, it also showed strong response to mean annual temperature. Null model analysis showed that deterministic processes were most important in shaping the phylogenetic community structure and assembly of diazotrophs. Diazotrophic communities of each sampling site tended to be phylogenetically clustered, while communities at high pH soils (pH 7.00 to 8.00) showed more clustering than those of low pH (pH 5.02 to 7.00). The soil pH is a main driver in structuring soil diazotrophic community assembly by regulating the relative importance of deterministic and stochastic processes in QTP alpine meadows. •Soil diazotrophic community assembly in alpine meadows was studied.•Soil pH is a main driver shaping diazotrophic community assembly.•Diazotrophic communities show more phylogenetic clustering at high pH soils.•Deterministic processes govern phylogenetic structure and turnover of diazotrophs.