Abstract: Magnetic field sensors have extensive applications in various fields such as resource exploration, industrial production, and geomagnetic detection. Traditional electric magnetic field sensors have good stability, but they have disadvantages such as susceptibility to chemical corrosion, high cost, large size, and poor resistance to electromagnetic interference. In contrast, fiber optic magnetic field sensors have advantages such as compact structure, high precision, small size, and strong resistance to electromagnetic interference. Existing fiber optic magnetic field sensors mainly include the following four categories based on basic principles: based on magnetostrictive materials, based on magnetic fluid materials, based on Faraday effect, and based on fiber lasers. Among these, fiber optic magnetic field sensors based on magnetostriction have more advantages in manufacturing processes and applications. This article introduces the development of fiber optic sensing technology for magnetic field measurement, analyzes the principles, advantages and disadvantages, and research status of four different types of sensors. At the same time, the application of fiber optic magnetic field sensors in geomagnetic monitoring was also introduced. Finally, the challenges and future development trends faced by fiber optic magnetic field sensors were analyzed.