A double-sided magnetic sensing composite structure is proposed using FeGa magnetostrictive films with variable cross-section on the top and the bottom of the AT-cut quartz wafer. The variable cross-section shape is utilized to reduce the demagnetization factor of the convergent area of the magnetic films, further increasing the strain in the electrode area. Meanwhile, the change in the strain direction along the wafer thickness direction can be avoided by adopting the double-sided composite structure. Therefore, the sensitivity of the resonant magnetic field sensor can be improved effectively by combining above two methods. The derived equations show that the sensitivity is positively correlated to the strain transfer coefficient determined by the composite structure, the piezomagnetic coefficient of the magnetostrictive material, and the thickness of the magnetic film, while inversely proportional to the square of the quartz wafer thickness. The sensor samples are fabricated by sputter deposition, with 1 μm thick variable cross-section FeGa films on the top and the bottom surface of the 200 μm thick AT-cut quartz wafer. Experimental results show that the Q value and sensitivity of the sensor are 5 489 and -0. 82 Hz / Oe, respectively. Therefore, when the thickness of the quartz wafer is reduced to 7. 5 μm, the sensitivity of the sensor can reach upwards to - 583 Hz / Oe. In addition, the sensitivity of the sensor can be further increased by using a higher piezomagnetic coefficient magnetostrictive material.