High native species diversity generally suppresses non-native invasions, but many ecosystems are now characterized by non-native assemblages that vary in species diversity. How this non-native species diversity affects subsequent invaders and its environmental dependence remain unclear. We conducted a plant-soil feedback experiment. In the conditioning phase, we created three diversity levels (1, 2, or 4 species) using six non-native species to condition the soil. In the responding phase, we planted these six species individually with soil inocula and exposed them to two watering treatments (well-watered vs drought). Under well-watered conditions, the non-native biomass increased with soil inocula generated by different non-native diversity. This biomass pattern was mainly related to arbuscular mycorrhizal fungal richness which increased with non-native species diversity. However, under drought conditions, the non-native biomass did not depend on soil inocula generated by non-native diversity. Our results reveal the crucial role of soil microbial legacies in driving the positive diversity-invasibility relationships of non-native communities and drought stress can eliminate these positive relationships. These findings provide an explanation for the commonly observed co-occurrence of multiple non-native species in nature, predicting an accelerating accumulation of non-native species in a benign environment, but not in a stressed environment.