In order to know the longitudinal flight quality of a launch aircraft during launch vehicle's separation, according to the change of aircraft's center of gravity, the longitudinal small perturbation state equations were built, and the time-domain method was applied to the simulation of long and short cycle modes, then the grade of flight quality was given according to criterion. The results show that the flight quality of short mode degrades from grade one to grade two, long mode only satisfies grade three during launch vehicle's separation. Obviously, the separation has serious effect on flight quality and it is necessary to improve flight quality and safety in the future.