Renewable energy is one of reliable approaches to provide electric power and fresh water in a solitary island. This paper analyzes the wind resource and power load from an island in South China Sea, proposes a wind-diesel-storage hybrid power system to provide electric power, and additional electric power is used to desalinate seawater. Wind-diesel-storage hybrid system power distribution design often relies on the experience, and this paper applies the improved PSO method to distribute the wind turbine generator number, diesel generator number and the storage battery capacity. The minimum of kWh cost is chosen as objective function, and the minimum loss of load probability (LLP) is chosen for constraint term. The energy is dispatched in order of wind turbine power, storage battery power and diesel generator power sequence during optimizing. Results show the improved PSO method has a little lower efficiency, but higher accuracy than standard PSO. This paper also predicts the power distribution results when the design parameters, such as diesel price, load, battery price, wind turbine price, are changed, and the change reasons are also analyzed. This paper can provide a reference for designing the wind-diesel-storage power distribution in a solitary island.