With the development of information technology, magnetic field detection plays a vital role in fields such as geological exploration. We propose a hollow-core anti-resonant fiber (HC-ARF) magnetic field sensor based on surface plasmon resonance, which is designed to excite surface plasma polariton modes by plating a gold film in the cladding tube of the HC-ARF, and we select an air cladding filled with magnetic fluid for magnetic field sensitivity detection. We analyzed the sensing characteristics of the sensor using the finite element method and studied in depth the effect of the gold film thickness on the magnetic field sensitivity. In our experiment, a maximum wavelength sensitivity of 101.92 pm/Oe was obtained in a magnetic field range of 60 to 180 Oe with a detection limit of 0.1962 Oe. The proposed sensor has the advantages of low loss and high sensitivity, which shows potential applications in the fields of geology, biomedicine, and military.