Abstract: To measure the isotopic abundance ratio of ~3He and ~4He in samples by massspectrometers is an important detection mean for helium isotope geochemistryresearch. A symmetrically arranged tandem ~3He/~4He mass spectrometer is describedin the paper. The front stage, used to analyse ~3He (including HD and H_3) and~4He, is a homogeneous analysing magnet with a bending angle of 90° and abending radius of 6 cm. The end stage, used to analyse ~3He, HD and H_3, is anon-homogeneous analysing magnet with a bending angle of 180°, a bending radiusof 15 cm and a magnetic field gradient of 0.75. Because of the use of the non-zero second order coefficient β and curved entrance face of the later magnet foreliminating second order aberrations, the resolving power of the system is notablyimprbved, and theoretically reaches about 3800.