Abstract: Gain modulation scannerless lidar is a new system of lidar. It is significant to investigate the range resolution of the lidar system. Based on the gain modulation lidar distance expression, the relationship between gain modulation lidar system parameters and range accuracy is being under investigation. Both photon noise with the weak echo intensity and gain saturation of the microchannel plate with the high range accuracy were taken into account. Curve between ranging accuracy and echo power was given. Gain modulation scannerless lidar system has a interval of echo power. By adjusting parameters to manipulate echo power into this interval, the highest ranging accuracy can be obtained.