Abstract: The webspinning sawfly (Chinolyda flagellicornis (F.Smith)) is an important forest pest insect injuring Cupressus funebris and Cryptomeria fortunei.It started to cause disastrous damage in Zhejiang, Sichuan, Hubei and Fujian provinces in 1988.A study on the regularity of the fluctuation of the natural population of the sawfly was carried out from1991 to 1997.The results showed that in the forest, the survival rate of the larvae was 60.4%,the regression coefficient r=0.9629, b=0.6559, in which the regression coefficient value (b) for the larvae at feeding stage was 0.5673, that for diapausing larvae was 0.3409,that for egg stage was 0.7989.Themortality of the larvae and the eggs were the first key lethal factor and the second one, respectively.The number of the diapausing prepupae in the forest excavated by wild boar to that in the forest not excavated by wild boar was 1:1.94～3.0.Other natural enemies had only a little effect.Themortality of the diapausing prepupae within 3 cmbelowtop soil in frost weather in winter at-2 ℃ was 43.8%～97.1%.The disastrous weather with hail significantly influence the survival of the sawfly.The index of the fluctuation of the natural population I=10.22 , indicating the population had a tendency of rapid increase.