The present invention discloses a wavelength selective switch and a method for controlling a spatial phase modulator in a wavelength selective switch. The wavelength selective switch includes: a first demultiplexing/multiplexing component, configured to split, an input multi-wavelength optical signal into multiple single-wavelength optical signals; a spatial phase modulator, configured to change a transmission direction of each single-wavelength optical signal included in the multiple single-wavelength optical signals, where the spatial phase modulator is further configured to split a first single-wavelength optical signal in the multiple single-wavelength optical signals into a first light beam and a second light beam, where the first light beam is incident on an output port, and the second light beam is incident on a monitoring port; a photoelectric detector, configured to receive the second light beam; and a performance monitoring component, configured to perform performance monitoring on the received second light beam.