Mg-based multilayers, including SiC/Mg, Co/Mg, ${\mathrm{B}}_4\mathrm{C}/\mathrm{Mg}$ , and Si/Mg, are investigated for solar imaging and a He II calibration lamp at a $30.4\mathrm{nm}$ wavelength. These multilayers were fabricated by a magnetron sputtering method and characterized by x-ray reflection. The reflectivities of these multilayers were measured by synchrotron radiation. Near-normal-incidence reflectivities of Co/Mg and SiC/Mg multilayer mirrors are as high as 40.3% and 44.6%, respectively, while those of ${\mathrm{B}}_4\mathrm{C}\text{/}\mathrm{Mg}$ and Si/Mg mirrors are too low for application. The measured results suggest that SiC/Mg, Co/Mg multilayers are promising for a $30.4\mathrm{nm}$ wavelength.