To measure magnetic field with Hall probe, the accuracy needs to be considered synchronously. Therefore, the improvements of measurement system are presented, including the magnetic needles and a zero-field shielding were simulated by OPERA. The design of magnetic needles with N-N type is significant that the misalignment of magnet could be calibrated. The doping area of Hall probe was sensitive, whose position should be defined before measuring. Also, a zero-field shielding is used to reset the value of magnetic field to zero. However, the data of simulation is reported in this paper.