Abstract: There exists ponding water during rainfall or irrigation inducing runoff into the unsaturated soil of farmland,and the soil chemical is lost into the surface runoff.A two-layer system model is established to describe the chemical transfer to surface runoff.An analytical solution for the model is obtained based on the chemical conservation laws and water balance equations,and the unsaturated soil experiment shows that this two-layer analytical model is reliable and reasonable.The effects of various soil properties on the chemical transfer to surface runoff are discussed by sensitive analysis of some parameters of the unsaturated soil in the model and experiments.It is found that higher adsorptivity of unsaturated soil will reinforce the ability of the soil chemical transfer to surface runoff,but the soil with greater bulk density decreases and higher initial volumetric water content of soil increases the soil chemical transfer to surface runoff.