The fluctuation in the number of signal photoelectrons will cause a range walk error in a Geiger-mode avalanche photodiode (Gm-APD) lidar, which significantly depends on the target intensity. For a nanosecond-pulsed laser, the range walk error of traditional time-of-flight will cause deterioration. A new signal restoration method, based on the Poisson probability response model and the center-of-mass algorithm, is proposed to restrain the range walk error. We obtain a high-precision depth and intensity merged 3D image using this method. The range accuracy is 0.6 cm, and the intensity error is less than 3%.