In this paper, we demonstrate a chip-scale optomechanical resonator driven into oscillation mode for new magnetic field sensing. The new sensing device based on nanofabrication uses the slot-type photonic crystal cavity for detection, which has the advantages of high sensitivity, high detection precision and frequency readout. Firstly, combined with the principle of the optomechanical system, the overall structure design of the optomechanical sensor based on Lorentz force for magnetic field detection is proposed. Then, the detection sensitivity of the designed optomechanical magnetic field sensor is analyzed. The simulation results show that the magnetic field detection sensitivity is 7.9Hz/,.. T when the input laser intensity is 199,.. W. Finally, the mechanical mode of the magnetic field sensor is simulated and designed, and the resonant data of the operating mode is obtained by testing. The results show that the new optomechanical magnetic field sensor has the characteristics of high sensitivity, low noise and can be widely used in future miniaturization, high precision, weak magnetic field and other applications.