Abstract: This paper theoretically analyzes the features of hot air drying system for grain, exergy reference point, enthalpy and exergy structure of drying chamber and exergy efficiency，in order to reveal the nature of energy consumption of grain drying system. The results shows that the moisture content of grain is a state function, establishes the exergy reference point of drying system for starting calculation, proposes exergy benchmark functions and plots the changing process of moist air on its psychrometric chart. The paper also reveals the matching relation between "quantity" and "quality" by analyzing the exergy efficiency. It lays a foundation for further study of the reasons and position of energy loss. Additionally, it provides scientific analysis methods to investigate the approach and the process design of energy efficient drying system and to develop reasonable evaluation criteria for the system.