A method and system, for flight management of an aircraft flying on a trajectory shifted with respect to a flight plan comprising a plurality of constrained waypoints, comprises a step of determining and displaying at least one point of the trajectory, termed decision point, beyond which the aircraft can no longer rejoin a constrained waypoint of the flight plan by determining a point of intersection between the trajectory and a rejoining trajectory steering towards the selected constrained waypoint, the rejoining trajectory complying with at least one predefined criterion.