Abstract: Underwater energy storage is receiving increasing attention to address the challenges of integrating marine renewable energy, represented by offshore wind power, into the power grid. Underwater pumped hydro storage (UPHS) is typical of these energy storage methods. However, the focus of existing research on UPHS is far from the electrical and energy field, and there is no in-depth analysis of capacity allocation and planning of real energy storage projects yet. To solve this problem, this paper researches the UPHS optimization configuration problem from the (quasi-) steady state perspective. Firstly, the structure of the offshore wind farm (OWF) and UPHS combined system is clarified, and a system model is established based on the principle of UPHS and wind turbine characteristics. After that, a mathematical model to derive the optimal capacity allocation is built to maximize investment return. Optimal capacities and real-time outputs of OWF and UPHS are solved under certain wind speed conditions. Finally, a comparative analysis of some key parameters is carried out to show their impacts on energy storage planning. Results show that UPHS can significantly improve the construction scale and investment returns of OWFs; meanwhile, the size and utilization hours also have multi-dimensional influences on the planning decision.