Making full use of solar energy is a feasible way to reduce building heating energy consumption in winter. A mathematical model is developed to investigate the dynamic heat transfer process in building external walls in the hot summer and cold winter zone of China by employing numerical simulation. Solar energy entering rooms through opaque walls is quantitatively analyzed by excluding the influence of outdoor air temperature on the room's net heat gain. The results indicate that solar energy entering rooms through opaque walls is of the same importance as the solar energy entering the room through a window for reducing building heating energy consumption. The solar absorptance of a south-facing room through the opaque wall can reach 17% when the heat-transfer coefficient of the south wall outer surface is low (i.e., under conditions of calm wind or high building density). With the consideration of solar energy entering rooms through the opaque walls, the reduction of the building heat loss index accounts for 20-80% and 10-45% of the total building heat loss index required by the national design standard (JGJ26, 2010) for the wall with internal and external thermal insulations, respectively. Therefore, a significant reduction in building heating energy consumption can be achieved by appropriate building layout such that the wind speed is reduced near the south walls. In addition, semi-empirical formulas for estimating the room solar absorptance through an opaque wall are given for the condition of consecutive sunny days in winter.