Unwanted cladding light in output of double-cladding fiber lasers has major adverse effect on the beam quality, spectral characteristics, and reliability and stability of lasers. It is of vital importance to strip the cladding light for engineering of the double-cladding fiber lasers. The thermal effect of high power cladding light strippers is studied theoretically and experimentally. The temperature distribution is simulated by means of computational fluid dynamics based on ANSYS. The experiments validate that the temperature simulation is effective and applicable for stripping power ranging from 15 W to 600 W. Furthermore, a kW-level cladding light stripper is optimized based on the thermal simulation. After optimization, the temperature of the stripper declines by up to 46 ℃, and the stripper can run stably at about 80 ℃.