Nowadays, more and more navigation systems based on the map of the environment have become mature enough to be used in practical application. Because the map is the basis of all these kind of systems, a 2D laser mapping system is designed to overcome some defects of the magnetic guide and RFID parking system used by the substation inspection robot now, and hoped to help the robot to fulfill the navigation tasks in the future application. The structure of the system is presents firstly, and then the algorithms for mapping are described in detail. Finally a prototype system is constructed, and an experimental test is carried out in a 500kV outdoor substation to verify the performance of the system. The experimental results show that the laser mapping system designed can build the map successfully, and the output map correctly represents the actual feature of the substation environment.