Thermal effects of high power double-cladding fiber laser severely limit the output power and beam quality. Firstly, temperature distribution in high power double-cladding fiber laser was analyzed by thermal conductive equation. Secondly, temperature distribution was simulated by finite element methods, at different conditions. Results indicate that temperature distribution was hardly affected by thermal conductivity of polymer cladding materials, so, when calculating the heat transfer coefficient of the core, the first and the second cladding can be seen as identical; the temperature distribution was great affected by heat transfer coefficient of cladding surface, increasing the heat transfer coefficient can reduce the thermal effects of fiber laser; the cladding's radius also can affect the temperature distribution. Conclusions are useful for designing kW level fiber laser.