No magnetization confirmation is required before the magnetic resonance imaging. In view of the complex background magnetic field and large interference in the environment where the magnetic resonance imaging equipment is located, the system has problems such as high false alarm, detection failure, a ferromagnetic detection system based on fluxgate sensor is designed. The system uses fluxgate sensor and second harmonic method to measure magnetic field signal, and the analog circuit is designed for phase sensitive detection. The closed-loop system improves the linearity and anti-interference through integral feedback to make the probe work in a stable state. Kalman filter is used to process the background magnetic field and magnetic anomaly signal to improve the signal-to-noise ratio, and the energy detector is designed to detect the magnetic anomaly signal. The test results show that the system can reduce the false alarm when the magnetic resonance imaging equipment is working in the complex magnetic field environment outside the magnetic resonance imaging room, and effectively detect ferromagnetic substances.