Numerical simulation of an amphibious vehicle's ambient flow field which accounts free surface effort is made by using RNG-turbulence model, volume of fluid (VOF) model and Pressure Implicit with Splitting of Operators (PISO) method based on multi-block hybrid meshes. The computed total resistance less than 9% difference from corresponding experimental data, the agreement is good. Then primary research has been done on optimizing the geometrical characteristics of the amphibious vehicle which influence the hydrodynamic characteristics, including the cavity of the wheel, cut angle in the bodywork, the state of the wheel and the leading board in foreside. The computational results show that the hydrodynamic characteristics of the vehicle which has been improved is better than that of the original, which diminishes the total resistance 69.2% at the speed of 10km/h.