Climate change is driving unprecedented shifts in global precipitation patterns, yet how these hydrological perturbations alter the relationship between plant diversity and microbial biodiversity, both of which play integral roles in ecosystem functioning, remains a key unresolved issue in biodiversity-ecosystem function research. Here we conducted a large-scale biodiversity experiment in a semi-arid grassland, manipulating both plant diversity (monocultures to 16-species mixtures) and precipitation patterns (50%, 100%, and 150% of ambient rainfall) to investigate how plant diversity modulates soil prokaryotic and fungal biodiversity under altered precipitation. We found that high plant diversity enhanced prokaryotic diversity, network complexity, and stability under the ambient precipitation, consistent with insurance hypothesis. However, these positive effects were environmentally contingent. Under increased precipitation, the positive effect of plant diversity on prokaryotic diversity was strengthened (+100%). Under decreased precipitation, plant diversity no longer correlated with prokaryotic diversity or network stability; instead, it increased prokaryotic network complexity, consistent with stress gradient hypothesis. For fungi, plant diversity showed no correlation with diversity, however, it induced a U-shaped relationship with network complexity under ambient precipitation. Under decreased precipitation, both fungal diversity and network complexity exhibited similar U-shaped patterns as plant diversity increased. Furthermore, we revealed that soil total organic carbon positively drove prokaryote, while nitrate nitrogen negatively regulated fungi. Crucially, precipitation changes reshaped the plant-microbe relationship by changing soil carbon-nitrogen dynamics. These findings advanced our understanding of climate-plant-microbial feedback mechanisms and provided critical insights for sustainable grassland management under global change.