Though the double cladding fiber (DCF) laser’s ability of heat dissipation is better than the traditional solid-state laser’s, the heat generation in double-cladding fiber laser is the bottleneck in the development of high power lasers. The distribution of heat deposition in the practical 400 W DCF laser is quantitatively analyzed for the first time. Based on the heat dissipation model developed in this work, in order to ensure the safe and stable operation of the kilowatt-level DCF laser, the convective coefficient near the pumping ends should be greater than 2.8×10-2W·cm-2K-1. The cooling device for high power DCF laser’s pumping ends was designed and successfully used in a laser system, and the output power of 1kW was obtained.