This paper proposed a method for the comprehensive thermal performance assessment （equivalent heat transfer coefficient） of building envelopes in existing buildings. This method is based on the index evaluation and equivalent energy consumption model, which integrates many evaluation indexes into an equivalent index. The thermal performance and energy-saving potentiality of the building envelope are evaluated by calculating and comparing the heat transfer coefficient of the two equivalent energy consumption models. It is shown that only measurement of indoor and outdoor temperature is needed, while measurement of every parameter in the envelope structure is not needed, which can overcome the difficulties in testing the existing buildings with complex structures. The proposed method can assess the total thermal performance of building envelope rapidly and evaluate the necessity of retrofitting before energy-saving renovation.