The priniciple of GPS emulator, used in emulational GPS nevigation system for UAV has been described in detail in this paper. By analyzing the basic work manner for GPS and its receiver, the relationship between WGS-84 system and other coordinates, the GPS emulator is produced. It converts the location information obtained by UAV fly simulating computer into corresponding GPS signal, which is attained by the controlling computer in UAV. The experiments show that it is qualified in simulation.