The operation range of spaceborne infrared detector was computed for effective detection and early warning of hypersonic flight vehicles. Considering the influence of the different transmittance of target-detector and background-detector, path thermal and dispersion, a new operation range model was given. Parameters in the new model were the function of the wavelength and had no concern with the range. The infrared radiation characteristics computation model of X-51A′s skin, nozzle and plume were given on basis of aerodynamics, engine′s after burning theory, plume flow field and its simplified model. The infrared radiation model in the sea background was established on the influence of the environment. It′s shown from simulations that the radiation intensity and operation range in the medium wave are larger than those in the long wave and they become larger with higher velocity. And the operation range becomes smaller when considering dispersion. It′s inferred that the detecting ability is better in the medium wave and the dispersion should be considered in real detection.