Planar solar concentrator can provide dynamic concentration ratio, which has attracted researchers’ attention.However, the decrease of the optical efficiency is not improved because of the rays-leakage from the lightguide.In order to hinder the decrease of optical efficiency from ray leakage, a new design method of the ray-leakage-free planar solar concentrator is proposed in this paper. According to the mathematical computation, refraction law and reflection law, the theoretical formula of the maximal ray-leakage-free propagating length in lightguide is derived. Furthermore, the mathematical model among the values of the parabola arbitrary coefficient，the vertex angle of air gap structure, the collector width, the collector height and the ray-leakage-free concentration ratio is established. The model of the proposed ray-leakage-free planar solar concentrator is simulated by the ray-tracing software. In consideration of Fresnel loss and material absorption, the concentration ratios of the proposed concentrators reach 698×, 857× and 1 032× with the corresponding optical efficiencies 88.2%, 85.3% and 80.2%, respectively, when the sunlight possess 0.27° divergence half angle; the concentration ratio increases further while the optical efficiency decreases slowly when the length of the lightguide exceeds the ray-leakage-free range.