Self-mixing interference in DFB-LD for fiber sensing application has been analyzed in this paper. Due to the characteristics of good model and narrow spectrum, the DFB-LD has the application potentiality in the filed of self-mixing interference, and optical communication technique has rapidly driven the development of optical fiber sensing technique. The combination of self-mixing interference technique and optical fiber sensing technique can satisfy the request of microminiaturized sensing device and the demand of interrogation of optical fiber. It is possible to form a novel optical fiber sensing measurement network, and the combination is helpful to the application at the aspects of avigation, industrial automation, medical examination, etc. In this work, based on the theory of coupled wave, the variation of laser output caused by self-mixing interference has been presented. For optical fiber sensing application, the self-mixing interference under the condition of transmitting the external optical signal by fiber has been analyzed. The influence from the variation of attenuation caused by the increment of fiber length and the reflectivity of the remote target to the output signal of self-mixing interference has been discussed in numerical simulation.