A method for assisting low-altitude navigation of an aircraft equipped with a system suited to determine a flight-plan ground trajectory based on intermediate points P at an altitude alt(P), and the aircraft's performance. The method includes the following steps: for each point P, calculating a safe altitude, to obtain a point P.sub.safe, calculating a safe profile formed from segments joining the points P.sub.safe, extracting summits S from among the points P.sub.safe, determining the aircraft's weight at these points S, for each point S, determining the maximum climb slope MaxClimbFPA and the maximum descent slope MaxDescFPA, defining two performance segments which have slopes MaxClimbFPA and MaxDescFPA on either side of the point S and calculating a performance profile formed from performance segments.