A detail examination of the truncation errors of the pressure gradient force is shown in a coordinate system when orography was introduced into the numerical weather prediction model. By means of theoretical analysis and numerical calculation, it was found that the accuracys of Corby scheme are differed greatly for different definitions of σ and vertical distributions of variables of the model. In order to suppress the errors, some suggestions about vertical discretization of the model and finite difference formulation of pressure gradient force are given.