This paper presents a high concentrating photovoltaic/thermal system with GaAs cells. The system consists of a concentrating device, a tracking device and a cooling device with GaAs cells. The concentrating device uses a plane mirror concentrating array structure, which can provide a uniform light intensity distribution and an adjustable concentration ratio. This system increases the electricity output and can collect hot water as thermal source. A dynamic model is developed to analyze the influences of different mass flow rate and the concentration ratio. The model shows that the instantaneous thermal efficiency of the system can achieve about 54.5% and the instantaneous electrical efficiency can achieve 25.8%.