Abstract: The biological characterisitics and occurrence regularity of Phassus excrescens were studied from 1997 to 2000 in the eastern part of Heilongjiang Province and Jilin Province.In this area,most of its individuals took two years to finish one generation,and over-wintered two times with eggs and larvae,respectively.At the end of June or the first ten days of July,larvae transferred to damage Fraxinus mandshurica. Feeding with artificial diet indoor,the larvae finished 6 instars before pupation.In artificial Manchurican ash forest,the larva mainly damaged the truck basal(below 20cm) or truck of 2 to 20 years old Fraxinus mandshurica. The occurrence amount of this species was closely related to the forest age and the stand structure.Usually,pure artificial Fraxinus mandshurica stands and Fraxinus mandshurica Pinus koraiensis mixed stands were damaged seriously,while Fraxinus mandshurica Larix mixed forests were damaged lightly.