The existing laser Raman gas detection (LRGD) system is affected by system noise, and the detection accuracy of low concentration gas is poor. In order to improve the detection accuracy of the LRGD system for low concentration gas, a laser Raman gas detection method based on multi-signal superposition and pseudo inverse method (MSSPI) is proposed. Specifically, for low concentration gas, the ability of resisting system noise is enhanced by Raman signal superposition. The signal detection module is used to detect the superimposed Raman signal. The Raman signal of the single gas is reconstructed by the pseudo-inverse method, then the gas concentration is calculated by the corresponding formula. The experimental results show that the LRGD system with the MSSPI method improves detection accuracy of the low concentration gas. The MSSPI method does not change the existing LRGD system, the calculation is simple and efficient, which reduces the detection error to a certain extent. The proposed method can be used as an auxiliary method for gas detection.