In cavity ring-down technique, a signal smoothing method based on spatial filters which have been used in digital image processing is presented for accurate cavity decay time extraction of low signal-to-noise ratio decay signal. Its smoothing procedure was derived and its smoothing efficiency was compared with other smoothing methods. Derivation showed the mean value spatial filter had the highest smoothing efficiency. The application of decay signal smoothing was also analyzed. The combination of the mean value spatial filter with the weighted least square methods was recommended when processing low signal-to-noise ratio decay signals. This method was tested in experiment and achieved almost the same results with the Levenberg-Marquardt algorithm.