Abstract: This paper discusses the analytic solutions of the diffusion equation.for themovement of water in unsaturated soil. Firstly, according to the experience relationbetween the diffusivity "D(θ)" and water content in soil "θ",and through variablesubstitution, the original diffsion equation is converted into the form easy to solve.Then, the variables separution method together with the Boltzmann transform methodis used to find out the solution of the new partial differential equation. So the analyticfunction of θ(x. t) is obtained, which is verified by the experiment of water flowthrough unsaturated soil.