For the development of offshore solar energy resources, a lightweight single-layer floating photovoltaic supporting structure is proposed in this paper. However, the environmental conditions of extremely shallow water with large tidal ranges brings great challenges for the design of floating offshore photovoltaic array mooring system. The time-domain coupled dynamic analysis is adopted to simulate the whole dynamic response of the floating photovoltaic array and mooring system and the influence of different mooring parameters on the motion response of module array and the tension of mooring lines are investigated under the environmental loads such as wind, wave and current. The results indicate that the motion response and mooring line tension of the floating offshore photovoltaic module array can be effectively improved by reasonable selection of mooring horizontal span, buoys location, floater net buoyancy and pre-tension. Based on this, the optimum design of mooring system is carried out and the mooring system design meeting the engineering requirements is given.