Aiming at the problem of low quality factor of the resonant sensor, which will cause high output signal noise and low sensing resolution, a high-quality-factor resonant magnetic field sensor with FeGa film and AT-cut quartz crystal composites is proposed. When excited with ac voltage, the AT-cut quartz wafer will vibrate in thickness-shear mode. The FeGa film is magnetized in the magnetic field, then the film is subjected to the force of the magnetic field, which is transferred to the quartz wafer through interlayer coupling. Due to the force-frequency characteristics of the quartz wafer, its resonant frequency will change. In addition, the proposed sensor has a particularly high quality factor because of the low internal loss and high Q value of the quartz wafer, the stable and low noise sensing output is achieved. Experiment results show that the sensitivity of the resonant magnetic field sensor reaches 0. 2 Hz/ Oe, its quality factor reaches above 46 000 and the power consumption is lower than 8 μW. In addition, the sensor has a wide magnetic field measurement range, and the maximum measured magnetic field reaches 1 200 Oe. The proposed resonant magnetic field sensor has obvious advantages of high signal-to-noise output signal and low power consumption, also has simple structure, and can be fabricated easily.