During precise measurement of high reflectivity, cavity misalignment has great influence on reflectivity measurement results of ring-down spectroscopy. According to propagation equation of Gaussian beam and overlap-add of signal, a theory model is set up to research the influence of detector on its ring-down waveform by using data fitting of method of least squares. Applying this model, it is simulated on the computer that ring-down waveform changes with various cavity length maladjustments or mirror′s tilt, and the analyses is in good agreement with the experimental results. Results show, in order to improve metrical precision, it is suggested that ring-down signal on the oscillograph should be applied to adjust ring-down cavity. From the results, two kinds of criteria for cavity adjustment methods are proposed. One is that total figure of ring-down waveform should be applied to adjust cavity length maladjustments, the other is that mutuality of adjacent pulse should be applied to adjust cavity mirror' tilt.