We proposed a method to form a flat transmitted serrated-phase (SP) high-contrast-index subwavelength grating (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 symmetrical deflecting gratings, the SP-HBS is extensible in size, and can achieve excellent splitting ability regardless 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.