Surface acoustic wave (SAW) sensors with ferromagnetic materials are used to measure magnetic fields or electric currents. The magnetic field sensitivities of SAW magnetic field sensors are essentially influenced by various factors. The sensing mechanism is complex due to the multiphysics coupling of the magnetic field, solid mechanics, and electric field. The magnetostriction effect, ∆E effect, and the third-order material constants are taken into consideration. The shape demagnetizing effect is reduced by increasing the length-to-width ratio and length-to-height ratio of a ferromagnetic film on an SAW resonator. The model is verified by experiments and accurately predicts the magnetic field sensitivities of SAW resonant magnetic field sensors. The factors affecting the sensitivities are investigated from the perspective of the sensing mechanism. A grooved sensing surface structure is explored for improved sensitivity. The results are beneficial to design reliable SAW magnetic field sensors with enhanced sensitivity.