Abstract: In order to make reasonable proposals on the magnetic design of 30 cm diameter ion thruster, the effects of magnet diameter and the spacing on the magnet strength and the confinement length of the discharge electron have been studied. By utilizing Maxwell-3D software, the magnetic field distribution was obtained with the permanent magnet on the cylindrical section oriented at 30, 60 and 90 to that on the conical section. According to different field calculated, several discharge parameters, such as electron temperature, ion density and ionization efficiency were calculated. The results indicate that when discharge voltage of 30 V，magnet pole length of 0.008 m and magnet spacing of 0.12 m are set, the confinement length of the primary is in the order of 50 m. When the angles between the permanent magnet on the cylindrical section and that on the conical section are 30, 60 and 90, the magnetic field contour inside the discharge chamber is in the range of 0.002-0.005 T. With the angle being 60, the magnetic field and filed free region are more reasonable compared to the cases of 30 and 90, when the electron temperature is about 2-6 eV, the plasma density is about (4-8)1017/m3and the collisional frequency rate of electron is in the range of 0.2-1.8.