<p>Available phosphorus (AP) deficiency in severe saline–alkaline soil adversely affects crop growth. To increase soil AP and promote crop growth, a phosphorus-solubilizing bacteria (PSB) named KGA3, which is a strain of <i>Bacillus megaterium</i>, was inoculated on maize; A field experiment was conducted with CK (without KGA3) and KGA3 treatment. Co-occurrence network (CoNet) was constructed to identify the keystone taxa, and redundancy analysis (RDA) was carried out to explore the relationships between keystone taxa and soil properties under different treatments to reveal the mechanism by which strain KGA3 promotes crop growth; Inoculation with KGA3 significantly increased contents of soil available nitrogen and potassium, activities of dehydrogenase and protease enzymes, and contents of the soil ions water-soluble K⁺, water-soluble Ca²⁺, and SO₄^2–. According to co-occurrence network, bacterial community structure was stable with KGA3 treatment. In addition, the Cyanobacteria was specific keystone phyla in inoculated KGA3; We demonstrated that <i>Bacillus megaterium</i> KGA3 has potential as a biofertilizer to reduce the application of P fertilizers as well as increase crop productivity under severe saline-alkaline soils in semiarid regions.</p>