In this paper, thermal behavior of GaAs/AlGaAs single quantum well (SQW) high power laser diode (HPLD) is modeled by means of finite difference method (FDM). A numerical model is introduced that calculates the time dependant axial variations of photon density, carrier density and temperature in the semiconductor laser, self-consistently. The dynamic response of the high power laser's back facet temperature and the influence of the surface recombination velocity on the temperature of both facets are modeled numerically. The two-dimensional temperature distribution in the Cupper heat sink is also demonstrated. In order to gain precise results we use the Fermi's Golden Rule to calculate the exact temperature-dependent gain function of the quantum well laser.