GIS system of forest fire alarm” whose basic units are the fire detectors dispersedly installed in the forest communicates wirelessly, and sends the forest fire alarm to command center through the Internet, and the core of the command center is the electronic map and its controlling strategy design. Combining with system functions, this paper introduced detector encoding, information format and related transmission, discussed the structure, elements, data of the electronic map and its drawing algorithm, and the simulation of the electronic map is realized.