The invention relates to the technical field of environmental geotechnics, and particularly relates to a pollutant diffusion coefficient inversion method based on a horizontal diffuser and a VS2DI program. The method includes 1) a step of selecting soil and measuring geotechnical parameters of the soil, 2) a step of preparing a solution and preparing a soil sample, 3) a step of slicing the soil sample, and measuring a pollutant concentration, and 4) a step of inversing the diffusion coefficient by utilizing VS2DI. The method combining the head-free diffuser and the VS2DI can conveniently and effectively determine a landfill liner head-free diffusion coefficient. Diffusion coefficients under different conditions can be obtained through changing temperatures, pH values, and related parameters, thus providing reference basis for landfill liner designing. Compared with traditional methods, the method is simple, easy in operation and accurate in result.