Abstract: A novel plasma mass separator is designed base on principle of magnetic mass spectrometry. In the axisymmetric structure orthogonal electric field and magnetic field, the different mass ions focusing in different position enter their respective collection devices to realize mass separation. An analytical equation for the motion trajectory of the ion in the separator is derived according to Hamilton principle, and a hybrid mathematical model of plasma beam trajectory by combining the electronic constant temperature MHD is constructed. Basing on the model, the focusing and mass separation of plasma beam are simulated with matlab. It is shown that when the shielding effect of the plasma sheath is ignored, the total force applied to the beam is equal to the electric field force, the spiral motion of ion will rotate around the axis of symmetry. When the shielding effect of the plasma sheath is considered, the electron transfer is under weak bound in small value of axial magnetic field. Electrons rapidly accumulate in the sheath, the electric field force is shielded to a large extent, the beam motion radius increases rapidly, ions arrive outer electrode before beam focusing. With the increase of the axial magnetic field, the electron conductivity is decreased, the electrons accumulation slow down, the shielding effect of the plasma sheath is weakened, radial and axial distance of beam focal position are obviously reduced. If the value of axial magnetic field is reasonable, the ions with different mass are separated in order of charge-mass ratio, and their focal position depend on the quality. Because the mass of different isotope ions is close, the distance of their focal position is small. The work is of great importance for the exploration of the electromagnetic control mechanism of plasma beam, and the results establish a theoretical foundation for the research and development of the high purity mass separation.