The clearance measurement environment of engine and gas turbine is harsh due to the high temperature， high pressure， and strong vibration. In such a measurement environment， the low-coherence heterodyne interferometric clearance measurement technique faces the problem of weak signal and the limited measurement range caused by the low Signal-to-Noise Ratio （SNR）. To address this problem， a method using differential detection is proposed to enhance the SNR. The theoretical measurement model of the proposed method is strictly established， and the model indicates the method has the advantages of improving the SNR and expanding the measurement range. To prove the feasibility of the method， an all-fiber clearance measurement experimental verification system is built， and a comparison experiment between single-ended detection and differential detection is carried out. The experiment result shows that， under the same measurement conditions， the differential detection method improves the SNR by 4.22 times and increases the measurement range from 10 mm to 20 mm. Furthermore， the measurement uncertainty of the system is analyzed. It is theoretically and experimentally found that， due to the scanning speed instability of the optical fiber delay device， the measurement uncertainty increases with the increase of the clearance. But within 20 mm， the measurement uncertainty is less than 15 μm.