The present disclosure discloses a controlling device and method for a Heating Ventilation and Air Conditioning system. The controlling device includes a building heat load module, a data collecting module, an optimization module, and a controller. A building heat load model is established according to building envelope data. Field information data and setup data are collected. The building heat load model, the field information data, and the setup data are integrated to produce optimal setup data by optimal computation. According to the optimal setup data, the HVAC system proceeds to modulate room temperature, room humidity, or gas concentration to achieve functions of maintaining indoor thermal comfort and energy conservation.