In order to measure error of exposed junction type sheathed thermocouple temperature test in the fire test, and analyze the influence factors of thermocouple temperature measurement， the threedimensional simplified geometric model and mesh model of the thermocouple were established, the heat transfer process was analyzed by numerical simulation and theoretical analysis, and the error of the temperature calibration in the fire test was studied. Compared the simulation results with theoretical analysis results, the simulation results were consistent with the theoretical analysis model based on a large number of experimental data, the simulation model could be used to analyze the error of the thermocouple. In addition, the thermocouple measurement influence factors were analyzed with simulation models.It is concluded that the thermocouple metal sheath diameter is larger, by the gas velocity, the smaller the effects; thermocouple measurement error is mainly affected by the influence of the diameter of metal sheath and the diameter of the thermocouple bead has limited impact.