The invention discloses an electron magnetic spectrometer for measuring a superthermal electron energy spectrum.The electron magnetic spectrometer comprises a magnetic field cavity and light sensing assemblies.Openings are formed in at least two adjacent side walls of the magnetic field cavity respectively and located in the side wall adjacent to or opposite to the side wall where an incidence hole is located.Each light sensing assembly is inserted in the magnetic field cavity through the corresponding opening, and is composed of an imaging plate and a supporting plate connected with the imaging plate.After the light sensing assemblies are inserted in the magnetic field cavity, a sealed space is formed by the imaging plates, the side wall of the magnetic field cavity and an upper cover of the magnetic field cavity.The electron energy detection range is obviously increased, the size and weight of the instrument are obviously reduced, and the magnetic field uniformity and detection accuracy are obviously improved.