As one of the key design parameters of Hall thruster, magnetic field indirectly affects the macroscopic performance of the thruster by directly affecting electron transport, neutral atom ionization, plasma distribution and other microscopic behaviors. At present, the study on the influence of Hall thruster magnetic field focuses more on the size and distribution of the magnetic field in the discharge channel, while the little research on the influence of the plume magnetic field on the thruster. Based on this, the effect of plume region axial magnetic field profile on the performance of Hall thruster is studied by using two-dimensional hybrid simulation. The research results show that the axial magnetic field gradient in the plume region has a significant influence on the thruster performance, when the magnetic field characteristics (magnetic field topology and magnetic field intensity) in the discharge channel remain unchanged. The potential drop in the discharge channel decreases with the axial magnetic field gradient in the plume region decreasing. However，the electric field in the plume region and the peak ion number density in the discharge channel increase with the axial magnetic field gradient in the plume region decreasing. Overall, the performance of the thruster improvement by increasing the magnetic field strength in the plume region. More specifically, there is a critical value of axial magnetic field gradient in the plume region. When the axial magnetic field gradient in the plume region is greater than the critical value, the thrust increases with the axial magnetic field gradient decreasing. When the axial magnetic field gradient of the plume region is less than the critical value, the thrust decreases slightly with the axial magnetic field gradient decreasing. The comparison of plasma potential, electric field, ion number density and ionization rate distribution under different magnetic field distribution in the plume region shows that the effect of plume magnetic field on thrust is to affect the distribution of electric field in space by influencing the mobility of electrons, thus the thrust will change due to electric field. The results of this paper will provide theoretical support for the improvement performance of hall thrusters and the design of magnetic fields.