Aiming at the power measurement of high power laser, especially the long-time measurement requirements of high power laser, a laser power measurement method based on water cooling was designed. The laser power was calculated by measuring the temperature differences between the cooling water inlet and the outlet with the thermistor. The three-dimensional finite element model of the heat path structure of the absorption cavity was established by using the finite element simulation software, and the cooling water paths of different calibers were simulated. The test results show that testing the power of high power laser with this method, the measurement relative error can be guaranteed to be within 1.6%.