Liquid crystal on silicon (LCoS) is a novel programmable diffraction optical element, which has been widely used in optical display, communication and storage etc. The diffraction of LCoS is analyzed and applied into 1 × N wavelength selective switch (WSS) as a wavelength selector, which has succeeded in switching the input wavelength to any output port. Based on the optical scale diffraction theory, the 1st diffraction distribution and efficiency of LCoS are discussed. The results show that the diffraction loss of LCoS is stable and lower than 5 dB when the grating phase order is greater than 10 pixels. Further, an LCoS-based 1 × N WSS is designed and the flexible modulation of channel space is realized. The WSS crosstalk is low and each output can be controlled independently. The total system loss is around 10 dB~14 dB.