The influence of thermal effect on the energy conversion efficiency of concentrating photovoltaic system for multi-junction GaInP/GaAs/Ge thin-film solar cell is analyzed experimentally. With the increment of operation temperature, the maximal energy conversion efficiency and optimal loaded resistor will be changed. Under the condition of operation temperature lower than 90℃, this influence of thermal effect is very small. However, when the operation temperature exceeds 90℃, the maximal conversion efficiency of the cells will decrease sharply, and contrarily the corresponding optimal loaded resistor will increase quickly. Then the system performance will degenerate badly and a thermal management will be necessary.