Simulation research helps to aid and optimize the designing of gain-modulated scannerless lidar, and helps to analyze the configuration and imaging principle. A theoretical model of the gain-modulated scannerless imaging lidar system is established. A simulation software for gain-modulated scannerless lidar is programmed according to the theoretical model. Simulation results and experimental results show that it is helpful for the designing and parameter optimizing of gain-modulated scannerless lidar.