This paper presents a design method of a natural light indoor illumination system using a planar solar concentrator as a daylighting module which can obtain high optical concentration ratios. In this design, the concentrator is composed of lens arrays, hemisphere-coupling structures, a lightguide, plate of beam splitters (PBS) and photovoltaic cells. The concentrated rays are divided into two parts by PBS, the visible rays for indoor illumination and the invisible rays for energy store. The converted electrical energy from invisible rays can drive the light emitting diodes (LEDs) at the entrance of the lightguide for illumination compensation in the condition of overcast and raining. Simulation results show that the proposed natural illumination system can achieve the illuminance more than 500 lx, which meets the commercial illumination requirement. In addition, the effects of collector width and parabolic coefficient on optical concentration ratio and concentration-acceptance product are discussed in detail. Finally, a prototype concentrator is constructed to demonstrate the viability of the proposed natural illumination system. The experimental measurements are in close agreement with simulation results. To the best of our knowledge, this is the first time that planar solar concentrators have been applied in the indoor illumination system, and our results can provide new insight into possible designs of novel natural illumination.