In view of the thermocouple time constant test, the semiconductor laser with 0.6 μs rise time and 500 W power is selected as the excitation source instead of the CO2 laser. By shaping the laser beam, 1050 ℃ step temperature at the measuring side of the thermocouple is achieved as the laser operates at 20 V working voltage and 20 A working current. The measurement results are processed by wavelet denoising, and the measurement model is established. The uncertainty of the measurement results is evaluated by analyzing its sources. The measured time constant is 1.3268 s, and the expanded uncertainty is 1.12 ms. The time constant of large-range thermocouples can be tested through increasing the laser power and producing a higher step temperature at the measuring side of the thermocouple.