An on-board system for evaluating the severity of a lightning strike, intended to be installed in the interior of an airplane and connected to an avionic system of the airplane. The system comprises four devices, a master device and three slave devices, each device comprising a magnetic-field sensor. The devices are placed pairwise symmetrically about a plane of symmetry of the airplane, vertically in line with the leading edge of the wing of the airplane and vertically in line with the trailing edge of the wing, respectively. The system allows an evaluation of the severity of a lightning strike to be obtained depending on the measured magnetic fields.