Abstract: According to the steady-state heat conduction equation, the temperature distribution in the high power double-clad fiber laser(DCFL) has been analyzed. The influence of fiber size, cooling-boundary condition and pump power on temperature distribution are discussed based on the analytic solution. The theoretical maximum output power with given parameters is obtained. The maximum power of a commonly used DCFL is limited by the melting point of the material and the non-linear effect. The bottlenecks can be easily determined so as to come up with effective improved shemes, such as weakening or eliminating the linear effect, raising the heat-resistant of outer-clad and improving the cooling method.