The mathematical model of surface temperature sensor in a step change surface temperature measuring device is established.The influence of the thermal conductive plate on the time constant of measured surface temperature sensor is analysed,through the mathematical simulation and calculation of surface temperature sensor at different time constants, it is concluded that when the time constant of thermal conductive plate is 20 times greater than that of the measured surface temperature sensor, it can be ignored that the influence of the thermal conductive plate on the time constant of measured surface temperature sensor.