An optical device based on Michelson interferometer configuration is introduced, which can be used for on-line film thickness measurement. Optical cement is used to splice the glass modules with different lengths and thermal properties, so that the device can make compensation to the environmental temperature variation. Additionally, the double-fiber collimator which is used to input and output the optical signal and the device containing the glass modules are a whole. So the device will not be disturbed by the environment compared to the other interferometers during testing. The stability of the device is tested by an Agilent wavelength measurement system, and the output interference signal shows the excellent performance that the 95% confidential interval of the variation of the free spectral range (FSR) is ± 0.0005 nm in the disturbed environment (while the acquisition resolution of the Agilent system is 0.001 nm). So we believe this device can be used for on-line interference measurement. To demonstrate the feasibility of the automatic measurement of this device in real- time, the thickness of the cover slice is measured, the result is (177.4 ± 0.7) mm, which is comparable with the commercial thickness measurement equipment.