We design a device system to measure the terrestrial magnetic field horizontal component by using mechanics and optics techniques. A coil will be in the situation of simple harmonic vibration under the action of torsional moment and magnetic moment. The relative distance between the two equilibrium position and the period of the simple harmonic vibration is measured. Then the terrestrial magnetic field horizontal component is calculated. This device has advantages 　　of simply structure， flexible operation，and accurate results. Moreover， it can achieve the dynamic real-time monitor.