Capability of the detector influences the figure of measured ring-down signal very much. 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 signal and its metrical result by using data processing of only fitting. The change of ring-down waveform is numerically simulated in different case by this model, and based on this, it is calculated that relation between capability of the detector and measured reflectivity of the cavity ring-down spectroscopy (CRDS). Results show that appropriate detector can improve repeating precise of measured reflectivity for the present data processing, but its metrical result of 0.990045 is different from the true reflectivity of cavity′s mirror, which is 0.999. In the end, it is studied that metrical result is changed with reflectivity of cavity′s mirror, and another data processing is analyzed. Two collating ways for measured reflectivity are put forward, one is correcting in theory according to the difference between simulated result and true reflectivity of cavity's mirror; the second is adding pretreatment of ring-down waveform before data processing of fitting, such as comb filter.