The present disclosure discloses a method and a system for determining an energy spectrum of an incident electron beam. The method includes obtaining a plurality of deflection currents of a beam deflection device; for each of the plurality of deflection currents, determining an energy range of an ejected electron beam, and determining a target current of a target generated by the ejected electron beam irradiating the target, wherein the ejected electron beam is emitted from an output of the beam deflection device after the incident electron beam enters the beam deflection device. The method also includes determining the energy spectrum of the incident electron beam based on the energy ranges of the plurality of ejected electron beams and the corresponding target currents.