Based on the equivalent heat transfer coefficient method and orthogonal experiments，a method for evaluating the thermal performance of various envelopes in existing buildings is proposed. The heat transfer coefficients of the external wall，external window，roof，and external door are integrated into an index（comprehensive equivalent heat transfer coefficient）. It is only necessary to measure indoor and outdoor temperature sequences of the existing buildings and geometrical dimensions of envelopes. The comprehensive equivalent heat transfer coefficient can be calculated，and the equivalent heat transfer coefficient of various envelopes can be obtained by orthogonal experiments and regression analysis. By comparing the equivalent heat transfer coefficients of various envelopes with heat transfer coefficient limits in standards，the thermal performance can be evaluated. To verify the feasibility of the method，a small experimental building located in Changsha is taken as an example. The equivalent heat transfer coefficients of the external wall，external window，roof and external door are1.62 W/(m2·K），2.84 W/(m2·K），1.25 W/（m2·K） and 3.14 W/（m2·K），respectively. Comparing equivalent heat transfer coefficients with actual heat transfer coefficients，the values are in good agreement，which proves that the method is feasible in engineering applications.