Abstract: A high differentiation ratio occurs once above 6 years of the 3rd generation of the Masson's pine caterpillar,Dendrolimus Punctatus,in the infested areas.It causes on early Autumn outbreak of caterpillar populations and killed large area of pine stands.Meteorological factors correlated significantly to high ratios are mean temperatures of August,the initial day stably over 10℃,and hours of sunshine above 10℃ through the year.Key factor to break the diapause of the caterpillar is the stimulus to the 4th instar of the long sunshine of mid to late August,which makes the differentiation increase substantially.Based on the computation of the data of the investigations for 22 years in Jinshishan Forest Farm and 7 meteorological factors,a mathematic model for prediction of the ratios of the 3rd generation was worked out as the following:Y_i=∑^M_j-1∑ⁱ_k-1cjkδ_i（jk） By time series method,the differentiation ratios for 1986 and 1987 are close to that investigated on site.