Horizontal detection fluorescence lidar is an effective tool to observe atmospheric organic aerosols, and the calibration of its system constants is the key to obtaining the concentration of organic aerosols with fluorescence echo signals by inversion. A calibration method for the system constants of horizontal detection fluorescence lidar based on the fluorescence-Mie scattering echo signal intensity ratio is proposed. The proposed method uses the fluorescence-Mie scattering echo signal intensity ratio attained by horizontal detection and the aerosol concentration data measured by the particle counter at fixed points to obtain the system constants of the lidar, and thereby reduce the influence of the system geometry factor. The accuracy and stability of the proposed method are verified by a comparison with the traditional calibration method in the numerical calculation. Its feasibility and effectiveness are proved by a practical calibration experiment with a fluorescence-Mie scattering dual-channel lidar for horizontal near-field detection. The results show that the proposed method not only can effectively reduce the influence of the geometrical factor on calibration accuracy, but also offers better stability.