An optical length-change measurement technique is proposed based on an incoherent microwave photonic filter (MPF). The optical length under testing is inserted into an optical link of a single-bandpass MPF based on a polarization-processed incoherent light source. The key feature of the proposed technique is to transfer the length measurement in the optical domain to the electrical domain. In the electrical domain, the measurement resolution is extremely high thanks to the high-resolution measurement of microwave frequency response. In addition, since the MPF is a single-bandpass MPF, the optical length is uniquely determined by the central frequency of the MPF. A detailed investigation of the relation between the center frequency of the MPF and the optical length change is implemented. A measurement experiment is also demonstrated, and the experimental results show that the proposed technique has a measurement sensitivity of 1 GHz/mm with a high length-measurement resolution of 1 pm in theory. The proposed approach has the advantages of high sensitivity, high resolution, and immunity to power variation in electronic and optical links.