Mechanism of corn plant residue (CPR) improving soil was studied through a soil culture experiment. After CPRs were applied into soil, contents of biomass C and N were augmented, cation exchange capacity of the soil, five-carbon sugar and six-carbon sugar were increased significantly, content of loosely combined humic substances, unitedly combined humic substances and tightly combined humic substances also were enriched. However, content of stably combined humic substances was reduced, and five-C sugar was apt to change. CPRs improved activity of soil colloid. Corn stalk is different from corn stubble in effect on improving the soil.