Taking hypersonic waverider for example, the infrared detectability of hypersonic object in rarefied atmosphere space is researched. Heat flux distribution of the stagnation point as well as radiation-equilibrium temperature is calculated under different flying conditions with engineering algorithms; then atmospheric transmittance is calculated by summation over small intervals. After considering both effects of radiation difference between object and background, and dispersion on detector′ s operating range, a new model for calculation operating range of infrared detedion system is built. By simulation, dispersion coefficient has varied with distance between object and detector; meanwhile, the higher the velocity of object is, the further the operating range is. When compared with long-wave spectrum, detector working at short-medium spectrum has better operating range. By field test, proposed algorithm has high accuracy when compared with existing models.