Different capability laser has different pulse width, which influences the figure of ring-down waveform in the cavity ring-down system (CRDS), and makes measurement result relative departure from true reflectivity.According to propagation equation of Gaussian beam and overlap-add of signal, a theory model is set up by data fitting to research the influence of pulse laser on its ring-down signal and its metrical results. The change of ring-down waveform is numerically simulated in different cases by this model, and it is also calculated how the measurement result varies with the true reflectivity. Based on this, it is analyzed that relation between waveform of the laser and measured reflectivity of the CRDS. Results show that current data processing can not acquire exact measured reflectivity. and a collating ways for measured reflectivity is put forward.