In order to simulate the transonic and subsonic modes using the scramjet engine performance computational model established by this research group which based on the integral equations, the residual equation is established by critical mass flow method with a independent variable of static pressure at the export of isolator in this paper. This equation which expresses the extent of thermal chock is iterated by Newton-Raphson method to simulate the working condition of the transonic and subsonic modes on which there is a critical cross-section and no thermal chocks, viz. a thermal throat, in the combustor. The results show that the distribution of one-dimension parameters with a thermal throat in the combustor can be simulated with this method. Meanwhile, this method is of high accuracy, fast computing speed and good convergence. It contributes much to the solving of the scramjet engine performance computational model.