Observation of the He-II spectrum at the wavelength of 30.4 nm, a key spectrum in solar spectrum, is of great significance in investigating solar activity and space environment. Multilayer film reflective mirrors are usually adapted in solar observation at extreme ultraviolet (EUV) wave range. Reflection at the wavelength of 30.4 nm of the multilayer films composed of Mg is studied. The multilayer film is designed using evaluation function expressing as highest reflectivity. SiC/Mg, B₄C/Mg, C/Mg multilayer films are fabricated by using direct current (DC) magnetron sputtering, and the multilayer structures are measured using X-ray diffractometer. The multilayer mirror of B₄C/Mg has highest reflectivity in theory but SiC/Mg multilayer film has the best quality and the highest reflectivity in practice. The reflectivity of SiC/Mg multilayer mirror was measured by the reflectometer in synchrotron radiation laboratory. At incidence angle of 10°, the reflectivity is 44.6%.