A liquid crystal on silicon, a wavelength selective switch, an alignment direction obtaining method, and a method for manufacturing a liquid crystal on silicon. The liquid crystal on silicon has a first pixel area, and a first liquid crystal located in the first pixel area. The first liquid crystal is deflected in a plane perpendicular to a first panel, and is deflected towards a first direction in a plane parallel to the first panel. An alignment direction of the first partial alignment film located in the first pixel area is deflected towards a second direction relative to a polarization direction of an incident beam, and the second direction is opposite to the first direction to reduce a loss of a deflected beam.