This paper describes a method of grid map building for indoor tracked autonomous mobile robot (AMR) equipped with 2D laser rangefinder and electronic compass. grid for the indoor environtment is built by the mobile robot. grid map is widely used in mobile robot navigation tasks such as robot localization and path planning. map building technology is indispensable for mobile robot to execute exploration task in unknown environment. firstly, different type of maps as presentation of environment model are discussed and compared; the tracked AMR's hardware architecture is introduced. secondly, robot's kinematics model is established; the main sensor's (laser rangefinder, electronic compass, edometer and infrared sensors) models are analyzed; the methods of data acquistion are presented. thridly, the process of building grid map is described in detail, in which multi-sensors data fusion is implemented. finally, maps building experiments is done in the corridor and the office of the laboratory. resultts demonstrate the map building method is effective and suitable for the mobile robot to perform complex tasks in partially known or completely unknown environments.