Abstract: Heat transfer coefficient of building surface is an important basic calculation parameter in building thermal engineering,energy saving and HVAC field. In order to investigate the dynamic changes of heat flow and heat transfer coefficients on the actual building surface,and to provide accurate basic data for the selection of heat transfer coefficients and the setting of building boundary conditions,a new type of ultra-thin heat flow sensor is used in this study,the direct heat balance method is adopted,field tests are carried out on the exterior walls of an office building in Xi’an in the heating season. The variation curves of radiation,convection and total heat flux,convection heat transfer coefficient（CHTC）and radiation heat transfer coefficient（RHTC）of the outer wall are obtained. The results indicate that the radiation and convection heat transfer coefficients of the external wall change from 2.19 to 11.32（W/m~2·K）and 0.15 to 6.58 W/（m~2·K）,respectively. The radiation heat flux of the external wall accounts for 80% of the total heat flux.In the low wind speed area of urban center,the value of CHTC is very small,which needs to be reconsidered. The relationship between CHTC and wind speed and turbulence is given for natural wind with low wind speed. The correlation between CHTC and turbulence is batter than relationship between CHTC and wind speed. For heavy envelope,the error of heat transfer coefficient is very small,but for the lightweight envelope with larger heat transfer coefficient,the fixed value may cause greater errors.