This paper proposes an improved optical path system to solve two problems caused by domestic traditional sulfur dioxide detectors: less optical path fluorescence and low acquisition accuracy. The point light source is collimated and concentrated using a lens, and a narrow slit is used to reduce interference of stray light, thereby improving the defect of the light intensity convergence effect on the conventional optical path structure and improving the detection precision as well. Zemax is used to simulate the new and original light paths. By comparing the point map and detection view, the root-mean-square radius and geometric maximum radius of the new light path are reduced to 35.439 μm and 50.194 μm, respectively, and the light intensity of the contact area between the gas and light source is improved. The experimental test shows that, compared with the conventional instrument, the sulfur dioxide concentration indication value error of the improved optical path system is reduced from 2% to 1%, improving detection accuracy. This newly developed optical path system can effectively solve above problems and is suitable for the design of sulfur dioxide detectors.