The cavity ring-down spectroscopy is a kind of measuring reflectivity technique. According to propagation equation of Gaussian beam and metrical principle of cavity ring-down, the theory model is set up for reflectivity measure with influence of length misadjustment Based on this model, it is numerically simulated that influence of length misacUustment of ring-down cavity on its output signal. The results show the excellent agreement between simulation and experiment, and prove that the theory model is accurate.