[Objective] Soil fertilities of seven substrate slopes in Xiangjiaba hydropower project area were measured in the years from 2005 to 2014. The changing trend of soil fertility was predicted to provide theoretical and technical support for slope vegetation restoration in large water conservancy and hydropower project. [Methods] Grey theory and logistic equation were integrated to establish a structurally sound and high-precision logistic model for forecasting the future trend of main soil fertility. The main soil fertility was decided by Grey correlation degree among various soil fertility factors in different slopes. [Results] Grey correlation degrees between microbial biomass and other fertility factors were all larger than the ones among other fertility factors, and thereby the soil microbial biomass can be regarded as the main soil fertility factor and denote the soil fertility level. Soil fertilities indicated by microbial biomass were predicted having an overall order in 2005-2040 that is natural forest(NF) >natural secondary forest(NSF) >vegetation-growing concrete gunning(VGCG) >thick layer substrate(TLS) >framed beams and soil covered(FBSC) >external-soil spray seeding(ESS) >discarded residue(DR). [Conclusion] Soil microbial biomass on different substrate slopes will eventually get stable with the time pass by. The VGCG slope's fertility level is the best.