High concentrating photovoltaic (HCPV) technology plays a more and more important role in solar power generation due to its extremely high efficiency. However, the efficiency of the HCPV module can be reduced by many factors. Especially, there are not enough researches and knowledge on the light intensity distribution and non-uniform illumination of different wavelengths of light concentrated by Fresnel lens. It is generally considered that the maximum power of multi-junction solar cell is achieved when the cell is placed on the focal plane of Fresnel lens. But it is proved to be incorrect by our research. When light beams of different wavelengths go through the Fresnel lens, their light spot distributions on the optical axis are not the same as those when they have different refractive indexes in Fresnel lens. At the same time, the triple-junction solar cell consists of three sub-cells which absorb light beams of different wavelengths respectively. Therefore, the performance of triple-junction cells would be influenced by the light distribution along the optical axis, this is exactly what we want to study in this work. The method of simulating the light tracing is used to calculate and analyze the light intensity distribution and non-uniform characteristics of different wavelengths of light concentrated by Fresnel lens. Combined with them from the circuit network model of a triple-junction solar cell, the electrical performances of triple-junction solar cell at different positions along the optical axis are studied. It is found from the simulation that the performance of cell does not reach the best state when cell is placed on the focal plane. The power of cell on the focal plane reaches only 0.41 W while the maximum point arrives at 0.69 W. The high non-uniformity of light on cell surface when cell is placed on the focal plane causes the decline of power. And an outdoor HCPV testing system with the ability to change the distance between Fresnel lens and the cell is conducted. The experimental results and the simulation results match well, therefore our simulation approach is verified. It shows that the module achieves the maximum power on either side of the focal plane, and the output power can increase more than 20% after optimization. It is a result after equilibrium between light intensity and uniformity on cell surface.