Abstract: Aiming at the power measurement of high power laser power, especially the long time measurement requirement of high power laser oven, a laser power measurement method based on water cooling is designed. The temperature difference between inlet and outlet of cooling water is measured by thermistor. The laser power is calculated by water temperature difference. The three-dimensional finite element model of the heat path structure of the absorption cavity is established by using the finite element simulation software, and the cooling water path of different calibers is simulated. The test results show that the relative error of this method less than 1.6%.