A method of optimizing a flight plan of an aircraft used in a flight management system of an aircraft is disclosed. This method makes it possible to constrain the real cost index of the mission executed to the optimal cost index CIopt predetermined by the aircraft operator, by acting on objectives ETAopt and EFOBopt of values of ETA and of EFOB to be achieved. Objectives are linked to this optimal cost index, in order to make the system return from a cost index CIm to the optimal cost index CIopt, while taking account of various constraints imposed by the ATC or the CTFM. Both constraints mext influence the cost index and constraints which limit the field of possible solutions.