A novel red phosphor Li0.5Na1.5SiF6:Mn4+ (LNSF:Mn) based on the unequal dual-alkaline hexafluorosilicate with superior optical performances has been synthesized via ion-exchange between [MnF6](2-) and [SiF6](2-) at room temperature. The composition and the crystal structure of the as-obtained phosphor LNSF:Mn were determined by energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD), respectively. The formation mechanism of the red phosphor LNSF:Mn has been discussed in detail. The phosphor LNSF:Mn exhibits good chromaticity properties and a quantum yield (QY) of 96.1%, which are better than the identified fluorosilicate phosphors Na2SiF6:Mn4+ (NSF:Mn) and K2SiF6:Mn4+ (KSF:Mn). A broad and intense absorption in the blue and a bright emission in red-shifted wavelengths make the phosphor LNSF:Mn a desired candidate for applications in warm white light-emitting diodes.