In order to improve the dynamic range of LiDAR and to address the issue of off-target measurements in high-speed scanning LiDAR systems when capturing long-range targets， a method that involves deviation of the detector from the focal plane is proposed. The off-target phenomenon of distant targets， the principle of radial and axial deviation of the detector in the focal plane， and the spatial energy distribution of the echo spot near the focal plane are investigated. First， the scanning speed， focal length， and defocusing amount of the laser and the radial deviation angle and radial deviation amount of the echo spot center relative to the optical axis are analyzed according to the target distance. Next， the effect of variation in echo spot size on the detector is analyzed when the relative position relationship between the image plane， the focal plane， and the detector changes. Subsequently， the energy distribution of the echo spot in the detector and the light-receiving efficiency are evaluated， considering the effects of the energy distribution of the Gaussian beam， the aperture diaphragm of the optical system， diffraction， and defocusing. Finally， the variation law of the light-receiving efficiency of the target echo at different distances after the detector deviates from the focal plane is verified using numerical simulations and experiments. The experimental results indicate that the farthest target can be measured from approximately 800 to 1 300 m by adjusting the radial offset under the 75 Hz quadrilateral tower mirror-scanning mode. By adjusting the axial deviation， the light-receiving efficiency of the 5 m target can be decreased by approximately 70%， whereas that of the long-range target remains basically unchanged. The proposed method solves the problem of off-target echo in long-range targets and improves the dynamic range of LiDAR.