High temperature strain gauge is the main measurement sensor of high temperature structure strength test, fatigue test and life evaluation. The accuracy of gage factor affects the authenticity of strain correction data in high temperature strain test. Based on the analysis of the principle of the gage factor, it is clear that the gage factor is the relationship between the output of the strain gauge and the strain. The influence factors of standard strain were determined through the model analysis that use the four point simply supported beam and pure bending moment steel beam to test gage factor. The influencing factors of gage factor calibration include the test beam size and the axial force of test beam. The test results of practical gage factor show that the factor of simple beam is slightly larger and the dispersion is obviously larger. The output of strain gage at high temperature is related to the load strain, the thermal output of strain gage, the size change of test beam, the change of elastic modulus and the change of friction force. The method of eliminating thermal output is discussed. Theoretically, the dimensional change and elastic modulus change of the simply supported beam at temperature can be caused error as high as 22%. The temperature influence error is very small because the steel beam is the rigid closed static mechanical balance system. Using self-developed free frame high temperature bare grid strain gage and high temperature ceramic adhesive, the theoretical analysis is verified by testing the sensitivity coefficient with the change of temperature, the theoretical analysis is verified by measuring the gage factor change with temperature which use the simply supported beam and steel beam.