Ranging performance is the key indicator for a ranging lidar system, and a theoretical ranging performance model can be used for optimizing systematic parameters when designing a lidar system. The fluctuation of the signal photon numbers introduces range walk error to photon-counting lidars that can be as high as tens of centimeters. In this paper, based on the lidar equation and the statistical property of photon-counting detectors, a theoretical ranging performance model for photon-counting lidars with multiple detectors is first derived. Next, a theoretical correction method for offsetting range walk error is proposed, as verified by experiments using a Gm-APD (Geiger mode avalanche photodiode) lidar system. The results indicate that multiple detectors are very useful to maintain a consistent ranging precision when the mean received signal photons are variable. With a 1600 repetitive ranging measurement, the new method can achieve a centimeter ranging accuracy, with a mean and standard deviation of the residual errors of 1.14 cm and 1.23 cm, respectively. This new method is potentially suitable for a satellite photon-counting lidar system such as ICESat-2, as only a certain number of repetitive measurements, the time tag, and a certain number of triggered detectors are required to offset the range walk error.