Abstract: Based on survey data of population density of grassland caterpillar over the years and temperature and precipitation with longterm observation in the Maqin County, the climatic factors, especially temperature and precipitation, how to influence on the grassland caterpillar was determined in this study by analyzing the relationship between damage of grassland caterpillar and climate factors. The results of this study showed that the grassland area damaged by grassland caterpillar was 14.67104 ha in the Maqin County of Guoluo Prefecture, and the maximum population density was 500 per square meter, in 1993 and was serious in 2003. From 1971 to 2007, the mean annual temperature of Maqin County showed an increasing trend, and the local precipitation showed a decreasing trend. The correlation analysis results showed that the population density of grassland caterpillar was not related with the average annual temperature, annual precipitation, average annual temperature and annual precipitation, but was significantly related with the precipitation and the average temperature and precipitation in September of the last year, indicating that the occurrence of grassland caterpillar was closely related to water and heat conditions in September of the last year and the appropriate temperature and precipitation encouraged grassland caterpillars to become pest damage. This study did not support the view in which the temperature was the limiting factor to caterpillar life cycle because the increasing temperature and reducing precipitation in recent years in Maqin County made the temperature not be a limiting factor, while the precipitation in September became the a key factor which affected eclosion, copulation and oviposition and incubation in the caterpillar life cycle.