Abstract: The mass resolution of the dual-focus mass analyzer is closely connected to the magnetic field stability at the working point of the mass magnet, which is an important component of secondary ion mass spectrometry. High accuracy measurement of the magnetic field at the mass spectrometer magnet's working point is a prerequisite for obtaining high stability of the mass spectrometer magnet at the working point. Based on the newly built multi-channel ADC+FPGA data acquisition platform, a multi-sensor fusion algorithm based on the Kalman filter is constructed to accomplish high precision magnetic field measurement by fusing the signals of four Hall sensors and one DC current sensor. The magnetic field measurement accuracy reaches 30×10⁻⁶ under 0.5 T field strength condition with the excitation current provided by Danfysik power supply, and the indexes are better than the measurement method using only a single type of sensor, which meets the magnetic field measurement requirement of the secondary ion mass spectrometer magnet.