On the basis of a large amount of experimental data and results from numerical Calculation, an exponetial function for the relation between the ray tube cross-section and Shock wave mach number is presented in this paper. With Sucn a faction, the Calculation of the ground Surface Peak Overpressure distribution in the mach reflection region is greatly Simplfied, and the results of the Calculation agree fairly Well With experimental data. This method will be rery useful in the design of hardened protection Structuses.