In the process of air conditioning, not only the thermal comfort but also energy saving should be considered. A method, which combines a least enthalpy difference（LED） and mixing air ratio based on the theory of enthalpy, can help to approach to comfortable zone for the air before coming into air handling unit (AHU). Then several mathematic equations of mixing process were derived based on laws of conservation of mass and energy. Eventually, the models of HVAC were established under SIMULINK environment. And the control strategies are validated in the aspects of improving comfortable effect and saving energy. It is found that LED based PID controller can achieve the requirements of better thermal comfort and energy efficiency.