In order to solve the problems of long cycle, high cost and inability to calibrate thermocouple time constants online in conventional calibration methods, an in?situ online calibration method of thermocouple time constants based on loop current step response (LCSR) was introduced, and the basic principles and calibration methods of the LCSR time constant calibration were emphatically discussed. A comparative study on the calibration of time constants using the hot air tunnel method and the LCSR method was performed in a laboratory environment. The experimental results show that the calibration period of the LCSR method is much lower than that of the hot air tunnel method, and the time constant calibration results are basically the same. Finally, the test application prospect of in?situ online calibration in the real environment of temperature distortion is discussed.