In current work, a magnetic surface acoustic wave (MSAW) sensor composed of FeCoSiB/SiO2 multilayer film is studied. The FeCoSiB/SiO2 multilayer film is deposited on a traditional quartz surface acoustic wave resonator. When the applied magnetic field changes, the FeCoSiB/SiO2 multilayer film affects the phase velocity of the surface layer of the piezoelectric substrate by the giant Young’s modulus effect, thereby changing the resonant frequency of the SAW. By optimizing the soft magnetic properties and designing the structure of the magnetostrictive multilayer film, not only the relationship between the total thickness of the magnetostrictive layer and the quality factor of the resonator is obtained, but also the sensitivity of the magnetic sensor (Δf/H, i.e., the variation of the resonance frequency when the unit magnetic field is applied) is improved by 2 orders of magnitude to 419 Hz/SymbolmA@T. On this basis, the mechanism of butterflylike and looplike Δf-H curves showing along the hard and the easy axis was studied. Because of its high magnetic field sensitivity and good directional selectivity, the MSAW sensor will be of extremely high practical value in the field of weak magnetic field (such as geomagnetic field) detection.