Abstract: The thermo-emf between the Cu and SnPb and that between the CuNi and SnPb are studied with a comparative method. The type T (copper vs copper-nickel) thermocouple is chosen as a standard thermocouple, as is widely used in spacecraft thermal vacuum test. The experiment temperature ranges from -195 ℃ to 75 ℃. Keithley 2750 data acquisition system is used to record the measurements of the thermo-emf, which includes the 2750 digital multimeters, 7702 connection boxes, E5810 gateway and computer with special measurement software. The cold junction of the thermocouple is ice water mixture and the hot junction is stainless steel cylinder or thermos. The temperature difference and thermo-emf of the tin-copper and those of the copper-Constantan are treated by curve fitting. The results show that the thermo-emf of the CuNi-SnPb is 92% of that of type T thermocouple, and the thermo-emf of the Cu-SnPb is 7.1% of that of type T thermocouple with the same temperature difference. In order to eliminate the thermo-emf caused by CuNi, the solder joint of the loop should be kept in the same thermal boundary conditions, and some measures are taken, such as coating the thermocouple with multi-layer insulation components or making sure that the connector is some distance away from the heat sink.