With in situ experiment,the soil organic carbon (SOC),microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were analyzed in Eucalyptus grandis plantations with different stand densities(A1:833 tree·hm^-2,A2:1 333 tree·hm^-2 and A3:2 222 tree·hm^-2) in Rainy Area of Western China.The results were given as follows:(1)SOC in every stand with different densities were higher in summer and autumn,lower in spring and winter,but seasonal changes was relatively stable.The annual average SOC was 22.54 g·kg^-1(A1),19.76 g·kg^-1(A2),16.84 g·kg^-1(A3),and the difference between the density reached significant level;(2)MBC and MBN exhibited a clear seasonal pattern in every stand with different densities,the same as the SOC,with the increase of the density,soil microbial biomass carbon and nitrogen content were less;(3)The fluctuations of microbial quotient were slightly in the plantations with different densities.The microbial quotient fluctuated between 2.30%—2.44%(A1),2.14%—2.39%(A2) and 2.47%—2.69%(A3),respectively.(4)The part of MBC(N) with SOC,available N(P,K) had a significant relation in different densities (P<0.05).Consequently,the seasonal changes of the E.grandis plantation SOC,MBC,MBN and the microbial quotient were relatively stable,but the different stand density had a significant effect on them,and with the increase of density,the content of soil organic carbon,microbial biomass carbon and nitrogen decreased in Rainy Area of West China.