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Human activities and climate change are key drivers of biodiversity loss in natural ecosystems. In particular, a growing body of evidence shows that grazing and altered precipitation patterns strongly affect grassland plant and insect diversity respectively, but their combined effects and impact on trophic interactions are largely unknown. In this study, a field experiment was conducted with different grazing intensities and irrigation to examine how plant diversity responds to variation in grazing and precipitation, which in turn influences the diversity of herbivores and predators in insect communities in a meadow steppe. Overall, grazing and irrigation increased plant biomass and decreased plant diversity, except for non-significant effects of irrigation treatments on plant diversity. Higher herbivore diversity in grazing treatments was mediated by plant diversity because path analysis revealed a positive relationship between plant and herbivore species richness. Increased predator diversity caused by grazing was mediated indirectly through changes in herbivore abundance. Our results indicate that plant-mediated changes in herbivore diversity are consistent with the Resource Diversity Hypothesis, whereas changes in predator diversity correspond with the More Individuals Hypothesis. Our findings highlight the importance of assessing both direct and indirect effects of grazing and irrigation on the diversity of guilds at different trophic levels within an insect community. Furthermore, understanding mechanistic responses to such changes could help us to better account for biodiversity loss in the context of grazing and altered precipitation patterns.