Abstract: The relationship between damage degree and yield reduetion of Small poplar borer was studied in Yanbei of Shanxi Province．The results Show that high growth of poplar tree is reduced by 17％ because of top damaged．The percentage of top damaged in forest can be determined by the borer nutuber per twig and the wood volume losses is estimated by the gall number per twig．The economic threShold for the Small poplar borer iS established respectively on the basis of high growth reduction and wood volume losses in poplar plantation．According to the benefit cost analysis，the economic threshold for high growth reduction and wood volume losses is respectively，9，16，23，30，39，47，57 borer per tree and 19，32，49，66，84，103，123 gall per tree for 2-8 years poplar plantation．