Abstract: The spectrum of the CO₂ laser deep penetration welding plasma and fiber laser welding plume is recorded by using a spectrometer. Both the relative intensity method and the Boltzmann plot method are used to calculate the plasma temperature. The results show that, for the same plasma, very different temperatures are figured out with relative intensity method, when different peaks are chosen. However, the calculated plasma temperatures are very similar with Boltzmann plot method when different spectrum peaks are used. Comparison of the results indicates that the measured temperature is more accurate and reliable with the Boltzmann plot method. Then Boltzmann diagram method is used to calculate the plume temperature, and it is found out that the continuous spectrum intensity of the fiber laser welding plume radiation line cannot be ignored, which is slightly different from the calculation of the plasma temperature.