Abstract: Isotopes in space contain a lot of scientific information, such as whether there is water on the surface of the lunar, and how the composition of water on the moon differs from water on Earth. Through qualitative and quantitative analysis, the unknown field of human beings can be further explored. Double-focusing magnetic mass spectrometer (DFMS) is a type of magnetic MS that can simultaneously focus the energy and mass of ions. DFMS is widely used in isotope detection, geological element analysis and other fields because of its large dynamic range and high resolution. But traditional magnetic MS is not suitable for space exploration mission because of its large size and weight. In this work, a miniaturized MattauchHerzog type of DFMS was designed and constructed. The size of the physical part of this instrument was 24 cm×12 cm×9 cm. The physical part included electron impact ionization source, electrostatic analyzer, magnetic analyzer and phosphor screen detector. The ion source used in this study was a classical Neal electron impact ionization source, which adopted a lightweight design with a size of 80 cm³ and a weight of about 90 g, and an alpha slits was adopted to limit the angle of ions beam. The electrostatic analyzer adopted a curved structure design, which was divided into upper and lower poles. The deflection angle of the electrostatic field was 31.8°, and the two poles were kept at a certain distance so that the electric field was evenly distributed. The magnetic analyzer was made of permanent magnet material, which had the advantages of magnetically stable and uniform intensity of internal magnetic. A layer of metal shell was installed outside the magnet to reduce the effect of the dispersion field. The deflection radius at the center of the magnetic analyzer used in this instrument was 62 mm, the ion deflection angle was 90°, and the magnetic field strength was 0.85 T. Furthermore, the phosphor screen detector consists of three circular microchannel plates (MCPs) to amplify the received ion electrical signals, and through the phosphor screen, the conversion of ion electrical signals to optical signals was achieved. In this experiment, nitrogen (m/z 28) was used to test the performance of this instrument. The results showed that the resolution at m/z 28 was 65, calculated by the ion imaging processing software. And the sensitivity was 3.28×10⁴ V/Pa, the mass range can reach up to 70 u.