Abstract: The Time Domain Reflectometry(TDR) used to estimate soil water content was based on the relationship of soil dielectric constant\|water content. Errors of Topp model, Mixing model and Diffusion model were analyzed with test results. Test results show that parameters of Mixing model and Diffusion model are too randomly to be determined, but parameters of Topp model with neglecting the forms of soil component can be used in engineering with proper revising in despite of the error in the model.