In view of the requirements of large aperture lidar system in pollutant detection, a multi-wavelength lidar optical system is designed theoretically. The system is composed of the main receiving system and the fiber splitting coupling part, and its working wavelengths are 355, 532, 1064 nm, and the detection wavelength is 632.8 nm. The main receiving system adopts Cassegrain structure, and the fiber splitting coupling part adopts transmissive form to introduce chromatic aberration. The clear aperture of the system is Φ680 mm, and the effective focal length is 1548.53 mm. Through the selection of glass material and the distribution of optical power, the achromatic equations are established by using the corrected relative partial dispersion P andAbbe number V, and the preliminary structure is solved by PW method. The optical system is optimized by Zemax software to meet the requirements, and the final optimization results are better than λ/15 (λ=632.8 nm), which indicates that the designed optical system can be used in actual production and processing.