We describe a compact beam splitter based on grating-assisted coupler which consists of Bragg grating sandwiched between two parallel waveguides on the silicon-on-insulator platform. The coupled-mode theory is an important method to analyze waveguide structure. The coupling effect is affected by the grating refractive index perturbation due to the phase mismatch between two waveguides with different widths and refractive indices. The power difference between the transmitting and the reflecting directions in waveguide A is nearly 0 when the Bragg wavelength is 1.3464 μm, the index perturbation is 0.245, the period of grating is 0.2 μm, and the distance of two waveguides is 1 μm. At this time, cross couple neighbor waveguides are significantly suppressed. Beam splitter based on grating-assisted coupler is very useful in integrated optical circuits and photonic network-on-chip.