In this study, the common three-channel spectrometer was improved and optimized to realize the simultaneous detection of ozone and aerosol concentration using differential absorption lidar. First, the initial structure of the spectrometer is determined, followed by the calculation and analysis of the position of the four channels using the raster equation. Then, the four spectral channels are controlled within a reasonable mechanical structure range by adding distance constraints using a spherical mirror and a holographic grating. Finally, a four-channel and low F number spectrometer system is designed. The system uses circular array to linear fiber to improve the reception intensity of the receiving system for atmospheric echo signals and realizes accurate detection for 266, 289, 316, and 532 nm echo signal strength. The results show that the proposed spectrometer system can be connected to a linear fiber with a 0.12 NA. The obtained spectral resolution is better than 0.5 nm and 1 nm at 266, 289, 316 and 532 nm, respectively, thus, meeting the spectral resolution requirements of lidar detection. Moreover, the spectrometer's radius of curvature of the exit slit and the center position of the circle are analyzed. Our findings show that the proposed design can realize the simultaneous detection of aerosol and ozone by lidar and can also simplify the system structure.