In order to solve the problems existed in the calculation of self heating in the active layer of semiconductor lasers, a new model for simulating the self-heating is introduced. The heat source induced by the carrier injection is analyzed. And the method for calculating the conductance of hetero-junction is also investigated to improve the precision of calculated joule heat. Edge emitting laser is fabricated, and the width of P-contact and stripe of fabricated laser are 10 μm and 20 μm, respectively. From the simulation result, the heat source density deduced from the traditional experience model is much lower than that from the optimized thermal model suggested. Thus a lower temperature rising is proposed in the experience model. By testing the shift of the lasing characteristics at different injected currents, the temperature of active layer is gained. Finally, the change of temperature with injected current obtained from simulation and testing is compared. The temperature rising gained from the experience model is lower than that of the experimental one. However, the new optimized model solves this problem. As a result of the optimized model, the maximum deviation between simulated temperature rising and tested one is about 0.2 K when the injected current is above the threshold current.