Multibeam LIDAR has been widely applied in the fields of unmanned ground vehicle, mobile measurement and robots because of its low cost, small size and capability of acquiring 3-dimensional distance of objects in the scene. To reduce occlusion and improve the density and coverage of point cloud, two or more LIDAR devices are integrated together to complement each other. As the installation position and attitude of the LIDAR are different, relative position calibration is a key step before fusing different laser data. In order to calibrate the relative spatial position relationship, a method based on coplanar constraints was proposed. Different LIDARs captured the range data of the same plane simultaneously. Although the range data had different coordinate systems, they represented the same plane. Relative positions between LIADR were initially obtained by fitting the common plane of multiple range data of different coordinate systems, and then optimized by L-M algorithm to enhance the calibration accuracy. This method is simple, accurate, and suitable for most LIDAR configurations in practical applications.