Cooling IR focal plane system is paraxially analysed with the optical invariant and first order parameters are proposed which quantify the concentrition and variation of narcissus on focal plane arrays (FPA). The ralation between NITD and parameters of environment and system is summarized based on the theory of thermal-radiation theory. The narcissus of an optical design in 3~5 μm with F-number 2.75, view angle 4° and effective focal length 90 mm is caculated and optimised through controlling its first parameters and using real ray trace as confirmation in CODE V, after which the narcissus effect drops by 70%. The distribution of narcissus is simulated on practical conditions in ASAP and the results are presented, it is shown that the NITD dropped down below the preset value after optimization and the narcissus is controlled effectively.