Using solar energy for space heating is an efficient and simply way to satisfy the energy demands of buildings. In this study, a typical office building is selected as a case model to obtain indoor air temperature characteristics with dual heat storage devices. By analyzing our solar heating system, a mathematical model of the system working process is set up. Using the software matlab / simulink for simulation, the indoor air temperature characteristics in 1 day are obtained. Simulation and experimental results show good consistency. And using the mathematical model, the storage tank size is optimized to search for the minimum size for the fixed building. Based on our analysis, the optimum ratio of storage tank A volume and collector field area is 0.11 m. This research can be a good reference for the design of the solar heating system.