In order to meet the requirement of low volume fraction SO2 gas detection in grid atmospheric monitoring, aiming at the key problem of optical path in modular application of ultraviolet fluorescence method, object telecentric optical system was combined with double-plane-convex optical system to apply the excitation and acquisition paths of SO2 detection module. Theoretical analysis and experimental verification were carried out by complementary advantages of multi-optical systems. Through the establishment of the model, the influence of optical system on signal-to-noise ratio of reaction chamber was analyzed. Combining ZEMAX software simulation, Monte Carlo tolerance evaluation analysis and indirect experimental verification of the whole optical system, the application of the optimized optical system was achieved. The results show that, the square of correlation coefficient of instrument linearity can reach 0.9999. The optical system has strong application value. It can provide theoretical basis and experimental data support for the design of modular optical system of ultraviolet fluorescence method.