The narcissus is an important parameter for evaluating the imaging quality of the cooled infrared optical system.The cause of narcissus for infrared optical system was analyzed.The method for controlling and analyzing narcissus in optical design software was discussed.A method of inverse path tracing rays was proposed for analyzing the magnitude of narcissus of each surface in the optical system.A cooled optical system working in the LWIR was designedwhich focal length is 120 mm.The method of inverse path tracing rays was used to analyze the magnitude of narcissus of each surface of the optical systemwhich was in accordance with the YNI and I/Ibar parameters given by the optical design softwareand the results calculated by the empirical formula.And the method of inverse path tracing rays for calculating narcissus is more direct.