An experiment was performed to investigate how the magnetic field gradient in strong field zone affects the discharge processes of Hall thrusters, such as the ionization and acceleration of propellants, for further research on optimal design theory of magnetic field in discharge channels of Hall thrusters. The results show that magnetic field strength has little effect on the ionization of propellants within the designed range of magnetic field gradient, but has significant influence on the acceleration of ions.With the increase of magnetic field gradient, the ion-beam energy distribution tends to be concentrated, and the thrust efficiency tends to rise. Finally, further increase of magnetic field gradient may lead to a series of physical problems,among whith such as the finite Larmor radius effect, the changes in the law of electronic conduction mechanism, and the magnetic field gradient drift are analyzed.