Ampere force is proportional to the strength of magnetic field, and magnetic field sensors based on Ampere force show good linearity. Based on principle of Ampere force and an orthogonal dual-frequency fiber laser, a compact and miniature fiber laser magnetic field sensor is realized by sputtering Au coating on the fiber laser surface. Alternative current is injected into both ends of the proposed Au coating fiber laser under a perpendicular magnetic field. The Ampere force generated is exerted onto the fiber laser to induce an additional birefringence into the laser cavity, which leads to the beat note frequency change of the laser and the magnetic field can therefore be measured. Results show that the proposed fiber laser magnetic field sensor has good linearity and capacity of resisting disturbance.