A piece - wise function in the dependence of burning rate on the radiant flux and its break point are important characteristics of burning of double base propellants under external irradiation. Using Fourier equation can analyze the burning characteristics, including characteristics near the break point. Considering the shielding effects of carbonous layer forming on the burning surface on the transmission of external irradiation, the burning characteristics of 1040, 1041, N -5 and N propellants were analyzed. One of main reason induced a piece - wise function and the break point is the loss of radiant energy before the radiant energy reaches the surface of condensed phase.