Abstract: The studies show that thermoperiod,as one of the enviromental factors,plays an important role in the photoperiodic response of the pine caterpillar. The critical photoperiod of the larvae treated with the temperature varying from 23 to 30℃ (average 27.5℃)is about 13 h 46 min,a 13 min longer than that in the constant temperature(27.5℃).Thermoperiods can not only make the larvae to have a longer critical photoperiod but also make larvae to prolong its larval developmental period and make larvae have a relatively higher diapause ratio. In Gaoan,Jiangxi Province,the investigations demonstrate that considering the coeffect of photoperiods and thermoperiods a more accurate model can be established through fixing the critical date of differentiation of the pine caterpillar,which is the key facfor in the predicting of the population dynamics of the pine caterpillar.