Applications of magnetometers are affected mainly by their sensitivities and detection bandwidths. Till now, the applications of the centimeter-scale optomechanical magnetometer have been still limited by those two factors. In order to improve its sensing performance in a low frequency regime of the alternating current (AC) magnetic field sensor based on centimeter-scale whispering gallery mode resonator, we design a new centimeter-scale crystalline whispering gallery mode resonator which has different relative distributions of the magnetostrictive material (Terfenol-D) and the optical material (CaF₂) from the unoptimized centimeter-scale whispering gallery mode resonator. Experimental results show that this new resonator is able to detect the AC magnetic field ranging from 6 Hz to 1 MHz, and a peak sensitivity of 530 pT·Hz^–1/2 at 123.8 kHz is achieved without DC bias field in a magnetically unshielded non-cryogenic environment. On condition that the optical quality factor is at the same level of 10⁸ and there is no DC bias magnetic field, the best sensitivity of the optimized resonator is 11 times higher than that of the unoptimized resonator, and the corresponding detection frequency band is expanded by 1.67 times, switching from the frequency band of 10 Hz–600 kHz to 6 Hz–1 MHz. Besides, the device only needs 100 μW light intensity to operate, which offers us a low optical power consumption magnetometer. Within the detection frequency band, the proposed magnetometer can detect both a single frequency alternating magnetic field signal and an alternating magnetic field signal covering a certain frequency range. It can detect 50 or 60 Hz alternating magnetic field signal generated by current in the wire so that the working status of the power system can be monitored. If the sensing performance is further improved, it may be able to detect the magnetic field signal at frequency in a range of 1 kHz–10 MHz generated by the partial discharge current and the extremely low frequency human body magnetic field signal located in a frequency band of 10 mHz–1 kHz. Further improvement in sensing performance is possible through optimizing the system noise and the magnetic field response capability of the device, which might allow the device to possess the applications in the fields of power system fault monitoring and medical diagnosis.