In order to in-situ acquire the thickness of non-transparent samples, such as metal strip, with high precision, a compact thickness measurement method of non-transparent samples with all fiber based on differential spectral interference has been proposed. Firstly, the basic principle of spectral interference and the specific implementation of absolute distance measurement are introduced. Then, a set of optical fiber differential spectral interference for non transparent sample thickness measurement is designed and built, and the principle verification experiment is carried out to realize the high-precision thickness measurement of standard gauge block. Finally, the measurement error of this method is analyzed, and the results show that the accuracy of this method can reach the sub micron level.