Based on the low-vibration cryostat, a Mach-Zehnder laser interference microscopic system used for parameter measurement of cryogenic targets is developed. With the consideration of light diffraction effect, a complete formula for describing the variation of optical-path difference with incident angle is deduced. In a low temperature environment and when the fuel solidifies or gasifies completely, the interferograms are acquired respectively with the existence of fringes at the target pellet center. With the fringe shape analysis method and the optical-path difference formula, the target pellet thickness and the fuel gas concentration within it are calculated, respectively. In addition, based on the non-destructive method, the real-time characterization of measurement results of gas concentration is conducted. The relative error between the results from the above two ways is less than 5%.