Abstract: According to experimental parameters, a theory model is made up to simulate measurement of reflectivity for length misadjustment of ring-down cavity. According to propagation equation of Gaussian beam, the influence of length misadjustment of ringdown cavity on its mode coupling is investigated, and the equation is given for relation between length misadjustment and output power, then the relation between ring-down signal and reflectivity is deduced from output characteristic of optical cavity. Applying this model, it is simulated that ringdown signal changes with length misadjustment of -10~10mm. The relation between metrical precision and length misadjustment is also numerically simulated. The analysis shows that the metrical precision drops rapidly with length misadjustment. To achiev