Pumped hydro storage (PHS) is the most mature storage technology with the largest installed capacity. However,its power regulation rate is slow,and its adjustment flexibility is relatively lacking. Electrochemical storage,which is currently the fastest developing storage equipment,provides capability of flexible power regulation. A photovoltaic-battery-pumped hybrid energy storage system is constructed in this paper. The goal is to improve the flexibility of PHS by utilizling the complementary characteristics of different storage facilities. The power regulation capability of the unit based on the power oscillation interval description of pumped storage is described，and further defines the flexibility of PHS according to the regulation of single unit and the unit-commitmen of multiple units. Considering the flexibility enhancing constraints and the demand to improve the operational lifespan of PHS,a capacity optimization strategy for the battery in hybrid energy storage system is established. A case study is conducted using a PHS in Shandong province as an example. The findings reveal that configuring an electrochemical energy storage system with a rated power equal to 10% of the PHS capacity can improve the flexibility of the storage system by about 40%,and reduce generation start-up times by 14%. However,under the current electricity pricing structure,the peak-valley price gap needs to surplus 0.5 yuan per kW·h for the electrochemical storage facility to become profitable.