Applying grating diffraction equation, multilevel diffractive characteristic of calibration lines of standard optical source was analyzed. Computer simulative calculation was completed on multilevel diffraction of standard optical source to solve intensity distribution of multilevel diffraction. Multilevel diffraction spectrum distribution of Hg lamp and He-Ne laser were measured, and the order of spectrum was defined. Fit curve was obtained by data processing and polynomial fitting. Infrared grating monochromator calibration was validated to be correct by measuring wavelength of a continuum wave CO2 laser and monitoring its wavelength to use a spectrum analysis instrument synchronously.