Abstract: The present disclosure relates to a wavelength selective switching apparatus and a related method thereof. The apparatus includes: at least one input port and a plurality of output ports; a dispersion assembly, configured to respectively disperse a received first-band optical signal and a received second-band optical signal into a plurality of first optical sub-signals and a plurality of second optical sub-signals in a dispersion plane; a first-band filter, configured to reflect the plurality of first optical sub-signals and transmit the plurality of second optical sub-signals; and a light redirection device, configured to make each of a plurality of sub-band optical signals to be emitted toward the first-band filter in a redirected manner. In a port plane, a plurality of optical sub-signals are respectively incident to the first-band filter at an incidence angle within a first predetermined angle range in a backward propagation direction.