The traditional SO2 detector in China has the problem of low fluorescence intensity in the fluorescence detection area of the gas chamber, which leads to the low monitoring accuracy of the instrument. In order to solve this problem, a new excited optical path structure was designed. In the light path structure, the point light source was collimated by a plane convex lens, and a narrow slit was used to eliminate the far-axis light in the vertical direction. The point light source was converged to the center of the gas chamber fluorescence collection area by a double convex lens, and the interference of stray light was reduced by the diaphragm. The ray tracing algorithm was used as a tool of strict vector analysis to simulate the designed optical path. The results show that the optimized optical path can reduce the energy loss of excitation light to 10%, and the dispersion spot is also greatly reduced. The experimental results show that the indication error of the optical path is changed from 0.34% full scale to 0.18% full scale. The precision of 100μg/L changed from 1.13μg/L to 0.53μg/L. The precision at 400μg/L changed from 2.26μg/L to 1.1μg/L. The two indexes have been improved, which can effectively solve the shortcomings of the traditional optical path.