The in-situ measurement of heat-transfer coefficient of external wall is the major content of building energy efficient measurement. A new test method and error assessment function were put forward. Computer simulation was conducted to analysis the influence of hot-box size, air temperature in hot box, structure and thickness of external wall on the result error to provide the theoretical guidance for test apparatus development. The results show that, size of hot-box and thickness of external wall is the major factors which decide the error in test. In-situ measurements were conducted according to the computer simulation results and show fine accuracy.