A wavelength selective switch (WSS) with hitless switching. The WSS includes the fiber collimator array, the focusing lens, collimating lens, diffraction grating, focusing lens, and attenuation reflection unit array. Each attenuation reflection unit has an interconnected transmission-type MEMS attenuator and a one-dimension MEMS reflector. The transmission-type MEMS attenuator is positioned in the front of the one-dimension MEMS reflector. The central axis of the transmission-type MEMS attenuator aligns and coincides with that of the one-dimension MEMS reflector, with the two central axes being glued together. The WSS of the present invention effectively utilizes the combination of a one-dimension reflector array and a transmission-type optical attenuator chip. With the use of one-dimension reflector array, instead of the known two-dimension reflector array, the complexity of design and manufacture is greatly reduced, thereby reducing the production costs of the switch.