The method allows stabilizing the rotation speed of a hydraulic machine with S-characteristics. It is implemented by means of a control loop feedback system having a controller for calculating an orientation to affect guide vanes of the machine. It includes steps of calculating a set of internal states associated with the operating point of the machine, establishing a linearized transfer function in function of the set of internal states, calculating characteristics parameters of the controller in function of the established transfer function so that the control loop feedback system is stable, measuring the rotation speed of the hydraulic machine, comparing the measured rotation speed with a target rotation speed, and adjusting the orientation affected to the guide vanes so as to reduce the speed difference between the calculated rotation speed and the target rotation speed.