Laser diode-based self-mixing interferometry (SMI) is a promising technique for noncontact sensing and its industry. In this paper, a phase unwrapping method in the several optical feedback levels is proposed based on displacement direction. A novel, to the best of our knowledge, method for evaluating the optical feedback factor C simultaneously with the linewidth enhancement is proposed by taking into account the effect of the accuracy of the linewidth enhancement $ \alpha $ on the evaluated method for C based on the differential feature of vibration reconstruction signal. Simulations and experiments are conducted to verify the proposed method.