In this paper, characteristics of IR radiation spectrum of target and decoy in space are discussed in detail using thermophysics theory based on their surface temperature field. The essential difference in IR radiation and mid-trajactory target is pointed out and a fundamental feature function (FFF) of IR target recognition is exracted. A 4-dimention feature vector for IR point target recognition is constructed with FFF. Emulator experiment of IR point target recognition meets with success by using this feature vector.