Abstract: A special investigative method in this paper called "trace investigation" was used to make life tables of natural populations of the small poplar borer Saperda papulnea L. in 3 types of forests, for 5 generations(1981-1985) It is time-saving，inexpensive，more precise and less tree-destructive to use this method to get the life tables．Moreover，the calculation is simple and easy．This new method through studying the borer’s biological and ecological characteristics was developed．By analyzing the life tables．Three conclusions could be made：(1)The borer eating the phloem around the twigs(1st period larva) had a higher mortality．Therefore this period was the weak link of its life cycle and the intestated control at this period should be strengthened．(2)The key factors influencing the population’s alternations were： In highly-productive man-made forests it was the squeezing af the first period larva and woodpecker’s pecking of the second period larva; In generally productive man-made forests，It was the desiccation of the first period larva； In low-productive forests，it was the Parasite of several ichneumon wasps and microbial pathogens． Strengthening these factor’s effects was the effective step to control the natural population of Saperda papulnea L．(3)The key factors’lethal ability had no relation to the population’s density． On the above basis the formula of forecast was set． NA=N₁×P_l×P₂×P₃×P₄. NA=Number of adults N₁=Number of total eggs P_l=Ratio of eggs’Stlrcicing． P₂=Ratio of 1St period larvas’surviving．P₃=Ratio of 2th period larvas’surviving． P₄=Ratio of pupas’surviving．The optlmum formula in different forests are： In highly productive forests， lgNA=0.95266·IgN₁P_lP₂P₃+0.506 In general productive forests,lgNA=1.1905·lgN₁P_lP₂P₃-0.398 In low Produetire forests．1gNA=1.1543·lgN₁P_lP₂P₃-0.3385