Aiming to the problem of determining unit capacity in small hydropower station synergistic expansion project, this paper calculates power generation of different installed partial project according to long series runoff data and based on a joint calculation mode of reservoir optimal regulation and economic operation in hydropower station. Considering other influence factors, such as the basic conditions of the original structures in hydropower station, the price of the hydraulic turbine group of the different programs, investment ratio and so on, the installed capacity of the hydropower station is determined reasonably. The model based on joint optimization to determine the installed capacity of hydropower station is proposed, it is more suitable for the actual situation of the power station, and the results is more accurate than the conventional method. Finally, some examples of the main steps are described to provide experiences for the technical reconstruction of the similar hydropower stations.