Abstract: By extending an interpolation mixed differential method for transonic small disturbed steady potential flow to the transonic large disturbed steady potential flow, we proposed an interpolation mixed differential method for solution of the exact equation for transonic potential flows in the local speed coodinate system. In numerical illustration of this method the pressure distributions of double arc airfoil and NACA0015 airfoil in symmetrical state are computed and compared with the data of experiments and the results of the computation for the double arc airfoil by the small disturbed mixed differential method. The comparisons are shown in a good approximation, therefore it is proved that the interpolation mixed differential scheme is stable and convergent. This paper has found a way out of the difficulty at Mach number near 1.