The invention, which belongs to the technical field of optoelectronics, discloses an optical beam splitter based on a non-periodic sub-wavelength grating and a design method thereof. The method comprises: establishing a three-dimensional coordinate system; forming a non-periodic grating basic unit by one-dimensional, two-dimensional and three-dimensional gratings; and extending the grating basic unit periodically to form an optical beam splitter having two deflection angles. Therefore, the light beam splitting in a vertical direction is realized; only power beam splitting is carried out; and the sub beam-splitting light can be kept in an incident light state. The grating basic unit formed based on the grating block structure is arranged periodically to form any one-dimensional, two-dimensional and three-dimensional grating; and the infinite extension is realized.