This Letter proposes an all-fiber self-mixing interferometric method based on laser feedback technology with long transmission distance, which has advantages of high sensitivity and compact structure. We theoretically and experimentally verify that the measurement distance of the self-mixing sensor is not limited to the coherence length of the solitary laser used. In the experiment, the velocity of a non-cooperative target was successfully measured with 40 km transmission distance. Therefore, the all-fiber self-mixing Doppler velocimeter has a great application prospect in the field of remote sensing measurement. Its unique flexibility can be applied to a variety of complex environments of non-cooperative target measurement.