The technology of grating side-coupling is often applied in such fields as coupling of light of single wavelength or narrow waveband, pump of fiber laser, integration of optical waveguide, but its application for broadband coupling of visible spectrum is rarely studied. Sunlight can concentrate and output at the edge of waveguides by integrating sub-wavelength gratings with waveguides, making it a novel solar concentrator. In this paper, different grating structures with the Finite Difference Time Domain(FDTD) solution software were simulated to obtain the optimal structure design. Besides, the angle of light for the coupling-efficiency was also discussed. The result demonstrates that the structures mentioned above all feature good diffractive efficiencies in broadband wavelength, among which the blazing grating reaches the largest efficiency, namely 48.8%. This kind of sub-wavelength gratings feature integration of small size, which makes it promising in absorption of solar energy, such as lumination, photovoltaic cell, etc.