Abstract:The relationships between the differentiation ratio of the third generation of the pine caterpillar Dendrolimus punctatus Walker and the environment,climate, population density and the development of the larvae were studied in this paper. The population density and peak day of hatching were proved to be the key factors influencing the ratio of the third generation. The ratio of the third generation in stands with high population density or early peak day of hatching will be higher than that with low density or late peak day of hatching. It will be significantly low when the needle loss exceeds 70% of the needles in a tree. There is not significant relationships between the ratio of the third generation and the ground vegetation, forest canopy and sides of low slope sides(<20°). The ratio of the third generation in a region is relatively stable in a given. Area in different years, while in different regions, it may change greatly according to the meteorological factors. A predication figure was established according to the peak hatching day of the second generation.