The structure and principle of double-channel elliptical crystal spectrometer are presented.The whole process of manufacturing dispersive element with natural muscovite is specified.The instrument was applied to diagnose plasma produced by laser on "XG-II" facility and the spectrum of flat Au was obtained.In order to analyze the reason of unsatisfactory pattern in which spectral lines interveined each other,X-ray diffraction experiment and observating surface of muscovite were conducted.The experimental results show that using such natural muscovite as dispersive element is the real reason for interlaced spectrum.