A signal monitoring method for a wavelength selective switch WSS, wherein after a wavelength division multiplexing WDM signal transmitted from an input port in a WSS passes through an incidence grating , light of wavelengths that is comprised in the WDM signal is incident to different positions or areas on a first optical engine ; and the method comprises: encoding (S101) a phase of the first optical engine based on the WDM signal, so that the WDM signal is split into a transmitted signal and a monitored signal after passing through the first optical engine , and the transmitted signal and the monitored signal are output at different emergence angles in a direction of an output port, wherein the monitored signal is input to a second optical engine disposed at an output-side grating , and energy of the transmitted signal is greater than that of the monitored signal, wherein the second optical engine is configured as a single MEMS micromirror; determining (S102), in the monitored signal, monitored light of a specified wavelength that currently needs to be monitored; and controlling (S103), based on an incidence angle at which the monitored light is incident to the second optical engine and an emergence angle at which the monitored light is output from the second optical engine , the second optical engine to rotate in a wavelength plane of the WDM signal, wherein the wavelength plane is a plane in which signal light of different wavelengths that is included in the WDM signal is spread out at different angles after the WDM signal passes through the incidence grating , so that the monitored light is output from the second optical engine at a preset angle, so that monitored light corresponding to different wavelengths is output in a time-sharing manner. encoding (S101) a phase of the first optical engine based on the WDM signal, so that the WDM signal is split into a transmitted signal and a monitored signal after passing through the first optical engine , and the transmitted signal and the monitored signal are output at different emergence angles in a direction of an output port, wherein the monitored signal is input to a second optical engine disposed at an output-side grating , and energy of the transmitted signal is greater than that of the monitored signal, wherein the second optical engine is configured as a single MEMS micromirror; determining (S102), in the monitored signal, monitored light of a specified wavelength that currently needs to be monitored; and controlling (S103), based on an incidence angle at which the monitored light is incident to the second optical engine and an emergence angle at which the monitored light is output from the second optical engine , the second optical engine to rotate in a wavelength plane of the WDM signal, wherein the wavelength plane is a plane in which signal light of different wavelengths that is included in the WDM signal is spread out at different angles after the WDM signal passes through the incidence grating , so that the monitored light is output from the second optical engine at a preset angle, so that monitored light corresponding to different wavelengths is output in a time-sharing manner. <EOS>