The time constant of the thermocouple is a key parameter of its dynamic characteristics. Obtaining temperature step by laser excitation is the main way to measure the time constant of the high-speed thermocouple. The heat transfer process of thermocouple excited by positive and negative step laser is compared and analyzed in this article. It is pointed out that the negative step is more consistent with the theory of time constant. The response model of thermocouple in the negative step of step modulated laser and pulse modulated laser is formulated. It shows that the pulsed laser can effectively solve the problems existing in experimental results of the step laser. The time constants measured under the same conditions are 73. 78 and 41. 34 ms, respectively. The limit performance of thermocouple response can be obtained by pulse laser test. Experimental results also show that the pulse energy has little effect on the measurement results of the time constant. Smaller thermocouples respond faster. The forced convection has a great influence on the measurement results of the thermocouple time constant.