A piece-wise linear function for the dependence of the burning rate on the radiant flux and its break point are important burning characteristics of double base propellants under external irradiation. The Fourier equation can be used to analyze the burning characteristics, but some early models resulted in discontinuous characteristic parameters at the break point. In this paper, a new model, where the loss of external irradiation energy was considered above the surface of condensed phase, would be used to resolve the contradiction. The burning characteristics of 1040, 1041, N-5 and N propellants were analyzed.