This paper investigates the thermal performance and energy-saving potential of the sunspace of an old apartment building in Qingdao through experiments and software simulations. This study found that different modes of user behavior can have a substantial impact on the thermal performance of the studied sunspace in winter. The bedroom with a non-ventilated sunspace showed a higher average temperature than the bedroom without the sunspace. However, the bedroom with the sunspace had more heat loss than the bedroom without the sunspace when the sunspace was naturally ventilated, especially at night. In Delta temperature (DT)-controlled ventilation mode, the heating load of a bedroom can be reduced by 2.94 kWh/m2 compared with the non-ventilated mode. Simply optimizing the roof configuration of the sunspace can significantly improve the heat gain of the non-ventilated sunspace and reduce the energy consumption for heating by 26 kWh/m2. Compared with the unoptimized sunspace, the optimized ventilated sunspace can reduce the heating load of the bedroom by 38.82 kWh/m2. In addition, the overheating of the room in summer can be solved by opening the exterior windows of the sunspace for ventilation during the day and leaving the door of the sunspace open at night.