Background and aimsPlant aboveground biomass production usually responds more strongly to rainfall addition than to reduction, however, whether plant diversity respond asymmetrically to rainfall addition versus reduction remains unclear. We aimed to assess the response patterns of grassland biomass and diversity to rainfall changes globally.MethodsWe conducted a global meta-analysis of 1,210 measurements from 169 grasslands field experiments on rainfall addition (R +) and reduction (R -).ResultsAboveground biomass increased by 0.72% per 1% R + but decreased by 0.48% per 1% R - , showing asymmetrical sensitivity, particularly in drylands. Soil cation exchange capacity (CEC) was the primary and positive regulator, amplifying biomass gains under R + . Conversely, species richness responded symmetrically to rainfall changes (0.12% increase per 1% R + vs. 0.13% decrease per 1% R - , with no significant difference between R + and R -). Mean annual precipitation (MAP) emerged as the dominant regulator, with sensitivity of species richness decreasing as MAP increases.ConclusionGreater CEC suggests greater soil nutrient availability, thus biomass asymmetry reflects the synergy between soil nutrient and R + . Plant diversity symmetry highlights the overriding role of precipitation in governing species richness responses to both R + and R - . These insights advance predicting grasslands forage production, economic benefit and climate adaptation amid increasing precipitation variability.