A secondary bedroom with radiation-fresh air composite system in Suzhou was taken as the research object. By using TRNSYS software simulation method, the influence of indoor setting temperature, radiation terminal water supply flow and water supply temperature on indoor thermal environment and indoor temperature lag time of radiation-fresh air composite system was studied under summer working conditions, and the appropriate precooling time of the system was explored. The results show that the indoor thermal environment changes greatly when the indoor setting temperature and water supply temperature change in summer, and the water supply flow has a certain impact on the indoor thermal environment. When the water supply flow increases from 220 kg/h to 260 kg/h, the time for the indoor temperature to reach the setting temperature is 1.0 h ahead of time. When the water supply flow is different, the time for the indoor thermal environment to reach stability is also different. The impact of water supply temperature on the indoor thermal environment is more obvious than that of water supply flow. Obviously, the lower the water supply temperature is, the shorter the time for PMV to reach class I comfort range is. In order to meet the requirements of indoor thermal comfort, the system can be opened 1.0～1.5 h in advance.