Abstract: In order to clarify the impact of cavity length change on measurement,taking the laser's line-width into account,the influence of cavity length change on the measurement of cavity ring-down in two limiting cases was analyzed and simulated by using cavity ring-down and multiple-beam interferometry theory,hereby the general cases were discussed.Analyses showed the instability chiefly changed resonant frequency of ring-down cavity,then varied the emergent light power and the feature of ring-down signal,which shouled bring serious measurement error.All of these will provide theory direction to measurement error analysis and improve measurement accuracy.