The traditional Differential Optical Absorption Spectroscopy(DOAS) method which is accurate and fast, has simple detection principle for H2S gas concentration. However, big errors occur in environments with low concentrations and short optical paths. In this paper, inversion of low concentration H2S gas is executed by using genetic algorithm based on traditional differential absorption algorithm. But the genetic algorithm is prone to premature convergence and thus falls into local optimum. The genetic algorithm after disaster optimization is designed to invert the H2S gas concentration. The results show that the method has high measurement accuracy for low concentration H2S gas, and combines with traditional differential absorption spectroscopy to obtain a wider detection range.