This paper proposes a single-layer sub-wavelength grating structure with beam convergence function and four-way beam splitting. The structure is designed and optimized using a rigorous coupled-wave analysis method and wavefront phase control principle, and the specific phase design rules are given. The simulation results of the structure using finite element software COMSOL show that the structure can realize four-way splitting under irradiation of transverse magnetic (TM) polarizes light at 1550 nm wavelength. Moreover, the optical power of each beam is equal and beam convergence is realized. The calculated total transmittance is 92.670%. Thus, the sub-wavelength grating structure can be applied in the important fields of optical communication integration and spatial optical coupling.