Semiconductor laser as an example,using the form of end-reflection coefficient and traveling wave rate equations,an analytical expression relating the average photon density inside the cavity and the output photon density of the semiconductor laser is derived to establish the relation between the average photon density inside the cavity and the output density of the laser in a self-consistent fashion of the rate equations based on mean-field approximation (MFREs) .The result is that ratio of the output field and the average field inside the cavity is related to the pumping current.The ratio cannot be considered as a constant.