The present disclosure discloses a method and device for channel switching, an Optical Network Unit (ONU) and a system for Time Wavelength Division Multiplexing (TWDM). The method for channel switching includes that: an ONU acquires channel information of a first TWDM channel, wherein the channel information of the first TWDM channel is used for indicating an uplink wavelength and/or downlink wavelength of the first TWDM channel; and the ONU tunes the uplink wavelength and/or downlink wavelength of the ONU into the uplink wavelength and/or downlink wavelength of the first TWDM channel according to the channel information of the first TWDM channel. By means of the present disclosure, the problem of high cost caused by specially building a standby channel for each TWDM channel in a system for TWDM Passive Optical Network (PON) to provide service protection is solved, and the cost of deploying the TWDM PON system is reduced.