Abstract: The temperature detection of laser is an important subject in the field of laser, which affects its applications in engineering directly. The XL-800WF multi-function laser welding machine was used to irradiate the thick steel plate and the melting graphs were recorded. From the micrographs, we can measure the melting spot size and calculate the solidification time. According to the principle of the spread of laser irradiation temperature, we have built the heat conduction equation under ideal conditions and simulated the temperature evolution. Combining with the solidification time, we have derived the theoretical temperature value of XL-800WF laser.