Since the size of modulated crystal is limited with size in the conventional photo-elastic modulation system, it’s a low spectrum resolution, so detection accuracy of gas concentration is not high. In order to improve the spectrum resolution of the system, an improved photo-elastic modulation system was designed. This method was carried out multiple reflection of incident light in the crystal. The optical path difference was increased by this method, so that a higher spectrum resolution was obtained. The entire system was constructed with the laser, photo-elastic modulator, polarizer, analyzer and photo-detector. Among them, a wedge angle (θ) was processed on modulation crystal. And a reflection film was plated on both sides; effective optical path length increase was implemented. Through the analysis the wedge angle (θ) was selected, and function on the optical path, the phase of modulation and the energy was calculated for the wedge angle. Ultimately, the appropriate wedge angle values were determined, and the corresponding optical path function expressions were given. In the experiments, the concentration of three common VOC gas was analyzed respectively. The concentration of sample gas was calibration by PTM400-VOC gas analyzer, and the results were compared with conventional photo-elastic modulation system. Experimental results show that gas concentration detection limit is reached to 0.010 mg·m-3 for improved photo-elastic modulation system, compared with conventional photo-elastic modulation system it’s an order of magnitude or more. The gas concentration detection error was 3.4%, and it’s also better than the traditional type. In summary, improved photo-elastic modulation system not only has the advantage of high static structural stability, but also a substantial increase in the spectrum resolution and detection accuracy of concentration.