This paper studied the variations of soil microbial biomass C, N, and P contents and soil hydrolase activities under different cropping modes, i.e., corn + sweet potato intercropping (CS), soybean mono-cropping (SM), continuous cropping of ginger (CG), and rice-milk vetch rotation (RM) , after ginger planting in the purple soil area at the lower reaches of Minjiang River. Ginger planting decreased the soil microbial biomass C, N and P contents significantly. The decrement of the soil microbial biomass C and N contents after ginger planting was lesser under CS and RM than under SM and CG, but the soil microbial biomass P content was in adverse. Ginger planting also decreased the soil acid phosphatase activity significantly, and the decrement was the greatest under CS but the least under RM. The soil invertase activity decreased significantly under CG, and the soil urease activity had a significant decrease under SM, CG and RM. After ginger planting, the soil urease and intervase activities under CS were higher, as compared with those under the other cropping modes.