Atmospheric nitrogen deposition has continued to increase in the Chinese terrestrial ecosystems over the past several decades. The input of exogenous nitrogen can alter the productivity and diversity of plant community, thus influencing the population dynamics of herbivorous insects. The grassland caterpillar (Gynaephora menyuanensis) is one of the main herbivorous insects of alpine grasslands in the northeast of the Qinghai-Tibet Plateau, and its outbreak can cause enormous ecological and economic losses. To explore whether increasing nitrogen deposition exacerbates or alleviates the outbreak of grassland caterpillar, we investigated the larval density of G. menyuanensis and their food quantity, food quality and habitat conditions by a nine-year nitrogen addition experiment in an alpine grassland on the Qinghai-Tibet Plateau. In the manipulative field experiment, a randomized block design was used with six blocks, and each block contained four levels of nitrogen addition including control (0 kg N hm^-2 a^-1), low nitrogen (25 kg N hm^-2 a^-1), medium nitrogen (50 kg N hm^-2 a^-1), and high nitrogen (100 kg N hm^-2 a^-1). The low, medium, and high nitrogen addition treatments were approximately 2.5, 5 and 10 times the ambient nitrogen deposition, respectively. Nitrogen was added using urea after sunset at the beginning of June, July, and August each year. Larval density of G. menyuanensis was investigated in sunny weather of July 2019. Our results showed that:(1) Nitrogen addition treatments altered the larval density of G. menyuanensis (ANOVA:F=3.29, P=0.04), and the larval density increased linearly with increasing rate of nitrogen addition (R²=0.31, P=0.005). Among them, the high nitrogen addition treatment increased the larval density by 85.4% (P<0.01). (2) Food quality (i.e., community-level leaf nitrogen content) played a main role in driving the responses of larval density of G. menyuanensis to nitrogen addition, while food quantity (i.e., aboveground plant biomass and aboveground graminoid mass) and habitat conditions (i.e., soil temperature and moisture and light condition) contributed less to the changes in larval density. Specifically, the pure effect of food quality accounted for 26.8% of variation in the larval density across different nitrogen addition treatments, and the joint effect of food quality and habitat conditions accounted for 18.3% of variation. The pure effects of food quantity and habitat conditions accounted for 1.7% and 12.2% of variation, respectively. Overall, these findings suggest that increasing nitrogen deposition may promote the outbreak of grassland caterpillars on the Qinghai-Tibet Plateau. They will also enhance our understanding of the interaction between plants and herbivores and provide practical advice on the adaptive management and pest control of alpine grasslands.