For the operating range calculation of staring IR imaging system,traditional operating range model was analyzed and a new model was proposed based on the target image's SNR detection criterion.The contrast of target and background and the IR image's clutter character were considered in this model,and the calculating formula of the operating range restricted by the complex background was derived.The experiment result of the ship-borne staring IR imaging system shows that the new model can be more accurately to calculate the operating range of IR imaging system in complex background.