Abstract: This paper aims to study on the improving effect of Chinese pine caterpillar’S frass on the soil fertility in the light of the frass that can compensate the tree growth in lagging time．The results show that during feeding of the Chinese pine caterpillar the fragmentation is important in material transformation；that the frass can ameliorate the physical and chemical characters；and that the frass applying can boost the number of soil microbe，especially the fungi which is three times as that of the check soil．The total microbiomass shows obvious increasing．The tannic acid and phenol in frass can promote the activity of nitrogenase by 3 times and every 100 gram dry frass soil can fix 22 milligram nitrogen by biological nitrogen fixation．

Key words: Trass, SoiI fertility, Material cycle, Compensating effect