In this paper, based on the dispersion of grating in monochromator, the cutoff wavelength of getting harmonics is judged on the appearance of 2 nd spectrum (wavelength of 69.13 nm) of the 23 rd high order harmonic (wavelength of 34.56 nm); Accordingly, the efficiency laser intensity of producing X spectrum is estimated in virtue of the theory of high harmonic which is 1.007×10 14 W/cm 2 and the most photon energy is 32.817 eV, which are the basic data of studying the origin and intensity of the spectrum in the experiment.