Abstract: The groundwater flow in unsaturated zone is a process of the two phase flow of water and gas.Based on the theory of multi phase flow,this paper deduces the two-phase flow dynamic model of the coupling of water and gas in unsaturated zone,and discusses the principle and move of the IMPES and the fully-implicit simultaneous solution method of the model.The paper thinks that because the Darcy's coefficient and the saturation are calculated explicitly,the stability of the IMPES solution method is weak,the precision is low,and it asks the time step of calculation is small;the fully implicit simultaneous solution method solves the equations of water and gas simultaneously and works out the values of the pressure and saturation implicitly at the same time,so it has high precision,is stable positively and is the important studying content of the model solution.