Abstract: In order to accurately test the thermocouple time constant, a high power semiconductor laser with rise time of 5μs and power of 500W was used as excitation source to heat the thermocouple. Theoretical analysis and experimental verification were carries out about time constant. Because of continuous output of semiconductor laser and stable uniformity of spot energy, the restriction of traditional excitation source mechanism of the formal test system was solved. The results show that, as the output power of semiconductor laser is constant, by closed-loop feedback control of laser power, laser step temperature rise signal is generated to ensure the uniform heating of thermocouple and obtain expected equilibrium temperature. Time constants of four different thermocouples were 2.806s, 3.094s, 2.229s and 2.457s. The feedback controller has strong robustness. The test system can stimulate the ideal step temperature rise signal to provide high quality excitation source for thermocouple time constant test.