A signal monitoring method and apparatus for a wavelength selective switch (WSS) are provided. The signal monitoring method for a wavelength selective switch WSS includes: encoding a phase of a first optical engine based on an input WDM signal, so that the WDM signal is split into a transmitted signal and a monitored signal after passing through the first optical engine; inputting the monitored signal to a second optical engine disposed at an output-side grating; and controlling the second optical engine to rotate in a wavelength plane of the WDM signal, so that monitored light of a specified wavelength in the monitored signal is output from the second optical engine at a preset angle.