Abstract: Hylobitelus xiaoi Zhang was a new outbreak pest which was found in Jiangxi province. To find out the key microclimate factor for the outbreak of H.xiaoi in plantation of Pinus elliotii, this paper used method in conventional observation to compare the microclimates in 6 plots, which had differently damaged by H.xiaoi, and analyzed the data with the grey relational grade analysis method. The results showed that the key microclimate factor was the land surface temperature, and then was the land surface light intensity. The relationship between them and the ratio of insectinfected trees were significantly negatively correlated and their gray correlations were both above 0.8. So the factors near the land surface were closely related to the outbreak of H.xiaoi.