Abstract: Thinfilm thickness is one of the most important parameters for holographic plate. Thinfilm thicknesses of two types of holographic plates were measured by Michelson interferometer which took the white light as its incident source. By analyzing the errors of the results, the relation of the measurement errors to thinfilm thickness and refractive index, and the applicable range of this method were studied. The result shows that with the increasing of film thickness, when it is from 8μm to 41μm, the absolute errors are ≤2μm with little fluctuations, but the relative errors decrease from 14.1% to 2.2%. The value of refractive indexes contributes to error propagation, and it is inversely proportional to absolute errors. In order to guarantee the validity of results, the thicknesses of films should be≥ 40μm when the refractive indexes are about 1.5. There are some advantages,such as a large measuring range, fewer experiment instruments and accurate result, when Michelson interferometer is used to measure some thicker thin films.