It is quick and accurate to on-line monitor the sample condition of laser cleaning by means of laser-induced plasma spectrum in air. In the present article, the echelle grating spectrometer was used to detect the plasma spectral lines induced by pulsed laser interaction with copper coin samples with or without contamination. The spectrogram showed that there were clear CuⅠ spectrum lines and air atom spectrum lines of NⅠand OⅠ. In order to eliminate the uncertainty of single measurement, the statistical regularity of NⅠand OⅠspectrum lines was analyzed. Their intensity distribution laws were consistent and their relative standard deviations were the same basically. So a single measurement spectrum could be used to monitor cleaning process. The spectra of copper samples with contamination consisted of many elements atomic spectral lines and continuous spectral lines. But there are CuⅠ spectral lines in the spectra of clean copper samples. As a result, the authors could detect the change of spectral lines to judge whether the laser cleaning samples were clean.