In order to meet the requirements of laser power measurement in the fields of beam synthesis, laser countermeasure and laser damage measurement of materials, a hundred-Watt-level thermopile laser power measurement instrument based on the Seebeck effect is developed. This instrument can work normally without bias voltage circuit, and can realize the continuous laser radiation power measurement with wide range and high stability. The power of the laser with the wavelength of 1064 nm is tested. In the power range of 15--250 W, the error between the test results of the thermo reactor laser power meter developed in this paper and that of the standard laser power meter is within the range of ±3%.