In the article, a study of sub-wavelength grating (SWG) nanostructures for broadband and omni-directional anti-reflection coatings (ARCs) on Cu2ZnSnS4 (CZTS) solar cells using the rigorous coupled-wave analysis (RCWA) method is presented. Various SWG nanostructures of different shapes and periodic geometry on CZTS solar cells are discussed in detail. The optimized reflectance decreased to 1.67%, and efficiency increased to 13.74%, accordingly. The omni-directional and broadband antireflections of the SWG nanostructures are also investigated. Under a simulated 1-sun condition and with the light incident angle increased to 80°, cells with SWG nanostructures enhanced the short-circuit current density by 16.5%. This considerable enhancement in light harvesting is attributed to the linearly graded effective refractive index profile from the air to the device surface.