In lithography, online spectral metrology of excimer laser lithographic light sources is used as the evaluation and monitoring the quality of the output laser lithography equipment, through the spectrum measurement we can know the running status of lithography equipment. Center wavelength and Full-Width-At-Half-Maximum(FWHM) are two important indicators of online spectral metrology. Traditional way of accurately measuring laser spectrum is to use a high resolution grating spectrometers. These instruments can provide accurate spectral measurement ,but are very bulky and expensive. Fabry - Perot (FP) etalon is based on the principle of multi-beam interference, high spectral resolution can be done, is a modern high-resolution spectroscopy indispensable instrument. echelle has big blaze Angle, can achieve high The blazed order, realize high resolution(lower than etalon). This paper introduces a method of using Echelle and etalon, through the analysis of the diffraction line fringes of ArF laser and a series of algorithms to deal with data, realize the on-board measurement of center wavelength and FWHM .