The research on thermal resistivity of high power fiber combiner is reported.The measurement method of thermal resistivity is studied.The thermal resistivity of high power fiber combiner of the Canadian ITF company is 0.32 ℃/W,0.15 ℃/W,0.29 ℃/W respectively under the conditions of passive cooling,active cooling and without the backward reflected light.Comparing the measured results between passive cooling and active cooling,it will be founded that the thermal resistivity under active cooling is lower.When the backward reflected light is eliminated,the thermal resistivity will be decreased also.The results are very important for the design of high power fiber laser.