A wavelength selective switch includes: N input ports, an input-side fiber array, an input-side collimator array, an input-side beam deformation and polarization conversion component, an input-side wave-demultiplexing component, an input-side switching engine, a focusing transformation lens group, an output-side switching engine, an output-side wave-combining component, an output-side beam deformation and polarization conversion component, an output-side collimator array, an output-side fiber array, and M output ports. The focusing transformation lens group includes two identical aspheric convex lenses that are placed in parallel, where a curvature from a center to an edge of a surface of the aspheric convex lens changes continuously. A spatial position of the focusing transformation lens group and the curvature from the center to the edge of the surface are so set that light with different wavelengths from the input-side switching engine is focused to a corresponding position of the output-side switching engine respectively.