Abstract: Based on the aerodynamics theory, the process of compressible gas behind the shock wave front flow through wall orifice is analyzed and the calculation method of internal average pressure is put forward, under an external chemical explosion condition. For brick masonry infilled reinforced concrete frame structures, the failure characteristics of exterior wall and the flying process of its fragments under internal and external pressure are studied. The calculation method of internal compression wave parameters is obtained and its efficiency is verified by the published experimental data. The influence of shock wave overpressure, orifice area and wall thickness on the internal rise time are analyzed by computational examples. The results show that in opening chambers, the internal rise time decreases as the orifice area increases. In close chambers，when the exterior wall failure happens, the internal rise time decreases with the decrease of wall thickness and the increase of external overpressure.