The sources of stray sight of space-based infrared detecting system are studied. They are analyzed and calcuated by use of blackboby radiation. The irradiance of the focal plane which arises from earth and atmosphere is analyzed by model established in TracePro. The projected solid angle subtended by the surface of earth and atmosphere to the camera can be known by the method of ray-tracing. The method makes the complex calculation become easy. Then the noises of detecting system are analyzed. The noise-equivalent-irradiance (NEI) of a certain Sofradir infrared-focal-plane-array is calculated and compared with irradiance induced by other stray lights. The result proved that the irradiance of the focal plane which arises from stray light is far bigger than the NEI of the infrared-focal-plane-array. Methods should be adopted to suppress stray light. The demand of rejection of the stray light from earth and atmosphere and the demand of the temperature of optical system are given. This result can be a reference for system design.