Using entropy density near event horizon of the non-stationary Dilaton-Maxwell black hole, the instantaneous emission flux is calculated, and we come to a conclusion that the instantaneous emission flux of blank hole in any direction at any time is always proportionate to the quartic power of temperature of the event horizon of black hole in that direction. It is found that the coefficient of generalized Stefan-Boltzmann is no longer a constant, but a dynamic proportional coefficient related to the rate of change of event horizon, the structure of space-time near event horizon and the radiation absorption coefficient of black hole. It shows that an inner relation between the gravitational field around black hole and its thermal radiation must exist.