The grassland caterpillar (Gynaephora menyuanensis) is an endemic pest in the northern Tibetan Plateau, which is a very susceptible region to global climate warming. Understanding how G. menyuanensis might respond to future warming under grazing conditions is critical to the sustainability of livestock husbandry in alpine meadows. We investigated the effects of diurnally asymmetrical warming (day/night 1.2/1.7℃) and grazing on the growth and development of G. menyuanensis in a Tibetan Plateau alpine meadow. Grazing significantly increased larval growth rate and pupal weight. The increase in female pupal weight was about two times greater than the increase in male pupal weight. Warming significantly accelerated larval pupation (female: 4.4 d; male: 7.0 d) and adult emergence (female: 3.6 d; male: 4.6 d). Male emergence under warming was significantly earlier (5.0 d) than female emergence. Combining the warming and grazing treatment, the larval growth rate and female pupal weight dramatically increased by 9% and 26%, respectively. The results of this study suggest that the fitness of G. menyuanensis may increase under future warming and moderate grazing conditions, posing a greater risk to the health of alpine meadows and livestock.