The heating system combining solar air collector with hollow ventilated interior wall (SAC-HVIW) can effectively extend the heating time. However, due to the large wall-window ratio of buildings on Tibetan Plateau, the strong solar radiation irradiating on the interior wall may influence the thermal performance of HVIW. In this paper, an experimental room is constructed to study the influence of external solar heat gain on the thermal performance of HVIW. Steady-state measurements are carried out by considering different ventilation rates, supply air temperatures and heat gains. Results show that the external heat gain has almost no effect on U-value, but it increases the heating capacity by increasing the logarithmic mean temperature difference (LMTD). For all cases, the heating capacity of HVIW is related to LMTD and supply air velocity, and U-value mainly increases with supply air velocity. Heat transfer of the interior surface of HVIW is dominated by forced convection which increases linearly with supply air velocity. The radiant heat transfer coefficient of the exterior surface of HVIW is not affected by the external heat gain with the mean value of 5.65 W/(m2 · K), while the convective heat transfer coefficient increases logarithmically with the external heat gain. The proportion of radiant heat transfer decreases as a power function with the increase of the exterior surface temperature. Measurements in this paper are used to evaluate the influence of external heat gain on the heating performance of HVIW, which is beneficial to the design of HVIW.