For the sensing applications based on the self-mixing interference technology of fiber laser, the self-mixing interference in the linear cavity fiber laser is theoretically analyzed through a four-mirror cavity model. The output power and frequency equation are deduced, and the behaviors of the laser under different optical feedback strengths are analyzed and simulated as well. The intensity of the laser is modulated by the length of the external cavity, and one fringe of the signal corresponds to the displacement of half wavelength of the target. Experimental setup is developed to validate the theoretical analysis. The obtained results provide both the theoretical and experimental basis for further studying self-mixing interferemetric sensing applications of fiber lasers.