Abstract: Pump source is an important component of fiber laser. Laser diode (LD) is the most common pump source for fiber laser, and temperature has significant influence on LD′s work. The thermal characteristics of LD as pump source under two kinds of working modes (continuous mode and discontinuous mode) is investigated by finite element method (FEM). A good agreement between simulation results and experimental data is obtained. The variation trends are similar and the maximum error is less than 5%, which prove the correction and feasibility of the simulation model. On this basis, the temperature field of pump source and the temperature variation characteristics of chips under 4 working conditions are simulated. A preliminary study on the improvement of heat dissipation structure shows that the changes in thickness of AIN submount and copper heat sink can reduce the junction temperature of chips at some degree.