A proposal for a high-precision accelerometer based on a silicon optical microelectromechanical system (MEMS) is presented in this paper. The device operating principle is that a sensitive mechanical structure senses external vibrations and then the vibrations of the mechanical component cause regular changes in the target modes in the optical system. Through a judicious design of the optical system, we obtain a wide photonic band gap (PBG) (approximately 1.1-2.75 μm) as well as two target modes in the PBG for modulation. Comparison with previously reported works shows that the proposed double-mode accelerometer displays better performance with respect to properties such as sensitivity and resolution. Accurate simulation results obtained by finite element analysis and rigorous coupled wave analysis have obtained the following results: an optical sensitivity of 4.42, a natural frequency of 1032 Hz, a bandwidth from 0 to approximately 900 Hz, a mechanical sensitivity of 0.242 μm/g, an accelerometer sensitivity of 1.017 μm/g, a linear measurement range of 0.517 g and a resolution of 933.7 ng (= 1 pm). Due to these optimized performance characteristics, the proposed accelerometer can achieve high precision detection in microvibration applications such as seismic detection.