In order to solve the high precision measurement requirements of the size and distance of the cargo for unmanned operation,a long arm 3D lidar system based on 2D lidar and a gimbal with step motor is designed,and focus on the calibration of lidar system parameters.To conquer the problems of ignoring the assemble errors with distance from the center of lidar to the center of rotation axis and specially points easily causing large calibration errors,a mathematical model of the lidar system is established.Then,by analyzing the internal connection among them,the decoupling method is used to separate the parameters to be calibrated,and the straight line fitting method and the double fitting plane overlap method are used to calibrate the unknown system parameters.It is verified by the methods with indoor calibration experiment and forklift carrying experiment,and the results show that the calibration parameters are accurate and reliable with measurement errors no more than 3%,which meet the requirement of unmanned operation of forklift.