The modification of exhaust gas temperature influenced by the variation of inlet air temperature is the most important work in the performance test of boilers. However, the difference between ASME code and GB standard always leads to misunderstandings. Based on analysis results of the modification method, it is concluded that the application conditions of this method is that: a good sealing of the air preheater should be maintained; the temperature of the inlet cool air should be adopted and the value of the air leakage ratio before and after modification should keep constant.