The effects of thermal mass on the indoor environment and energy consumption of convective air systems (CASs) have been well studied. Few research refers to the corresponding effects in the zone with radiant heating/cooling system. This article aims to study the effects of thermal mass in external walls on the transmission load and energy performance in the spaces with active cooling surfaces, compared with those in identical rooms equipped only with equivalent CASs. This study is based on the investigation of energy performances in an assumed typical office building using the EnergyPlus simulation software. The weather conditions in four typical days during the cooling season in two cities (i.e., Beijing and Nanjing) are considered. The inside massive layer has positive effects on indoor temperature performance and instantaneous transmission load, but little impact on energy consumption. The peak cooling load in the zone with heavyweight can be shifted in a proper operation strategy. Although the cooling surface reduces the effect of thermal mass to radiation heat transfer and enhances the conduction heat gain, a structure with an inside massive layer is recommended in the zone with cooling surface.