Abstract: In view of the dynamic variation of unit efficiency coefficient and its value in the middle and long-term power generation dispatching of the reservoir supported by the tailwater downstream,this paper starts with the reservoir water level,tailwater level and power generation flow of the reservoir,the key factors affecting the comprehensive efficiency coefficient under different water conditions and their influences are analyzed. A case study is carried out combined with the hourly operation process of the Three Gorges Hydropower Station. The results show that the ten-day comprehensive efficiency coefficient has a dynamic change,the difference between the maximum and the minimum is9.77%（8.71% in flood season）,and there is also a certain inter-annual difference（the minimum in 2017 is 4.44% more than that in 2018）.A further analysis shows that the value of ten-day comprehensive efficiency coefficient is smaller in the period of abundant water,the power generation flow is the main influencing factor,the tail water level takes the second place,the reservoir water level ranks lastly,the value is larger in the period of moderate or small discharge,and the power generation flow is the main influencing factor,the reservoir water level takes the second place and the tail water level ranks lastly.