The invention relates to a method for determining the energy distribution of a proton beam, said method being used as a diagnostic means for the protons produced in plasma inertial confinement or Z-pinch plants, which result from the interaction between plasma and high intensity laser beams, existing in cosmic radiation, generated by medical equipments for proton therapy, as well as the monitoring of environments subjected to ionizing radiation, such as space stations, spaceships and spacecrafts. According to the invention, the method consists in using a package of sheets, preferably of CHO, without being limited to this type of plastic, the thickness of each sheet being smaller than the width of the Bragg peak, defined as the difference between the maximum distance and the minimum distance of the proton path through matter, where the energy loss is 1/2 of the maximum value.