Wavelength switching method, device and system

A wavelength switching method, device and system, the wavelength switching method comprising: an OLT (110) transmits a wavelength switching command to an ONU (120), the wavelength switching command being used to indicate that the ONU (110) switches from a source wavelength channel to a target wavelength channel, and the wavelength switching command carrying a start time t0, t0 being used to indicate that the ONU starts to switch from the source wavelength channel to the target wavelength channel at t0; the OLT (110) starts to transmit downlink data to the ONU (120) via the target wavelength channel during the time T starting from t0, the time T being the time required by the ONU (120) to switch from the source wavelength channel to the target wavelength channel. The wavelength switching method, device and system are used to shorten the time required for wavelength switching.