We explored arbuscular mycorrhizal fungi (AMF) in the rhizosphere soil under Robinia pseudoacaciaL., Leguminosae (black locust), and evaluated the relations between their diversity and soil and climatic factors in the semi-arid Loess Plateau of northwest China. A total of 23 AMF species were identified at six sites. Of these, six belonged to the genus Funneliformis (Glomeraceae), five to Rhizophagus (Glomeraceae), three to Glomus (Glomeraceae), two to Acaulospora (Acaulosporaceae), two to Claroideoglomus (Claroideoglomeraceae), two to Septoglomus (Glomeraceae), one to Ambispora (Ambisporaceae), one to Scutellospora (Gigasporaceae) and one to Sclerocystis (Glomeraceae). The species that occurred in the largest proportion of soil samples were Rhizophagus intraradices (75%) and Funneliformis dimorphicum (66.7%). Soil-available potassium was positively correlated with mycorrhizal colonization and species richness. Spore density was negatively correlated with soil pH, whereas the Shannon-Wiener diversity index was positively correlated with total potassium. Most of the soil and climatic factors measured affected the AMF indices directly rather than indirectly. The relative abundances of 13 AMF species were strongly affected by precipitation, temperature, total nitrogen, ammonium nitrogen and available potassium. This suggests that AMF species are diverse and Funneliformis was the most common and dominant AMF genus in the rhizosphere soil under black locust in the Loess Plateau. Precipitation directly affected AMF status and diversity in this distinctive semi-arid ecosystem.