Abstract: With the urgent need of energy saving and consumption reduction in today's world, the topic of increasing the energy conversion efficiency of ventilation fans has been attracted a lot of attentions, it has become a key issue in the field of ventilation. The understanding on the evolution mechanism of irreversible energy loss in impeller is the premise and basis for realizing the efficient energy conversion of ventilation fans. At present, the irreversible energy loss mechanism in impeller is still lack of research. Therefore, the internal flow field of the contra-rotating axial fan at different flowrates is obtained by numerical simulation and experimental methods. A theoretical model of irreversible energy loss in impeller of ventilation fans is established based on entropy production theory, and the relationship between the irreversible energy loss in impeller and the flow field parameters is clarified. A quantitative analysis is conducted on different types of energy loss in impeller, and the spatial evolution law and causes of energy loss are clarified by combining with the flow characteristics in impeller. The results show that the entropy production method is reliable in calculating the irreversible energy loss in impeller. Direct viscous dissipation loss and wall friction loss are important components of energy loss, while turbulent dissipation is the main cause of energy loss, accounting for 60% to 80% of the total energy loss. For the front impeller, the energy loss caused by turbulent dissipation reaches the minimum at 1.0 Q_BEP, while the energy loss of the rear impeller increases with the decrease of flowrate. The energy loss is mainly concentrated in the region of Span=0.6−1.0, reaching 70% of the total energy loss at optimal flow condition. The spiral vortex caused by blade tip leakage flow and overflow of blade leading edge, backflow in impeller, flow separation at blade pressure and suction surface, and blade trailing edge wake will cause energy loss. The high energy loss region caused by flow separation and wake is relatively small, and the energy loss is relatively limited. However, the vortex and backflow significantly affect the flow in impeller, ultimately leading to a significant energy loss in the region near the blade tip. The research results can provide a reference for the evaluation of irreversible energy loss of ventilation fans.