Abstract: China has many cascade hydropower plants in her river basins,and the upstream and downstream plants in the same basin often belong to different market players. Due to the complex coupling relationship between upstream and downstream plants, when the cascade hydropower plants belonging to different entities participate in the day-ahead market bidding respectively,the clearing results of upstream and downstream may not match,resulting in the decrease of hydropower revenue and the increase of spill flow. Alliance bidding fully considers the hydraulic coupling relationship between upstream and downstream plants,and therefore can produce a well-matched bidding decision,which can improve the revenue of cascade hydropower plants in the day-ahead market and reduce the spill flow. In this paper,a bidding strategy optimization model of cascade alliance is proposed,in which the optimal bidding strategy of cascade alliance and the clearing of day-ahead market are taken as the upper and lower levels of the model respectively,and then the bilevel model is transformed into a monolayer mixed integer linear programming model by Karush-Kuhn-Tucker（KKT） conditions and dual theory. In addition,the Shapley Value is introduced to allocate the revenue of multi-operator cascade alliance. Finally,the effectiveness and rationality of the model are verified by simulation.