We proposed a method to form a flat transmitted serrated-phase(SP)high-contrast-index subwavelength gra-ting(HCG)beam splitter(HBS)for all dielectric materials,which is to alternately arrange two kinds of grating bars with a phase difference of π.Compared to the typical linear-phase(LP)HBS,which consists of two sym-metrical deflecting gratings,the SP-HBS is extensible in size,and can achieve excellent splitting ability regard-less of normal incidence or small-angle oblique incidence with large deflection angles,higher diffraction efficiency,lower energy loss,and higher tolerance of fabrication accuracy.Furthermore,the incident light can be split in half at any part of the SP-HBS,and the output beams of light maintain the original shape.In this Letter,we designed an SP-HBS with a 44.8° deflection angle and a 90.28％transmissivity.