To improve the accuracy of displacement measurements in self-mixing interferometry (SMI), a multiple self-mixing interferometry (MSMI) process that involves phase modulation is proposed. The phase is modulated using an electro-optic modulator that is placed in the external cavity. Phase demodulation is performed using the quadrature demodulation algorithm. In theory, the displacement reconstruction accuracy of MSMI is better than that of SMI, and this is demonstrated using simulations. A series of experiments are also conducted to confirm the effectiveness of the proposed method. The method is shown to meet the requirements for real-time high-precision measurements. In addition, the quadrature demodulation technique used does not involve complex computations.