Global warming and changes in precipitation patterns can critically influence the structure and productivity of terrestrial ecosystems. However, the underlying mechanisms are not fully understood. We conducted two independent but complementary experiments (one with warming and precipitation manipulation (+ or – 30%) and another with selective plant removal) in a semi-arid grassland on the Loess Plateau, northwestern China to assess how warming and altered precipitation affect plant community. Our results showed that warming and altered precipitation affected community aboveground net primary productivity (ANPP) through impacting soil moisture. Results of the removal experiment showed competitive relationships among dominant grasses, dominant semi-shrub and non-dominant species, which played a more important role than soil moisture in the response of plant community to warming and altered precipitation. Precipitation addition intensified the competition but primarily benefited dominant semi-shrubs. Warming and precipitation reduction enhanced water stresses but increased ANPP of dominant semi-shrubs and grasses, indicating that plant tolerance to drought critically meditated the community responses. These findings suggest that specie competitivity for water resources as well as tolerance to environmental stresses may dominate the responses of plant communities on the Loess Plateaus to future climate change factors.