A wavelength tunable optical transmitter apparatus, comprising a plurality of N multi-longitudinal mode lasers, a cyclic wavelength demultiplexer/multiplexer, and a reflector, wherein an output end of each of the multi-longitudinal mode lasers is connected to a respective one input end of the cyclic wavelength demultiplexer/multiplexer, an output end of the cyclic wavelength demultiplexer/multiplexer is connected to an input end of the reflector, and an output end of the reflector outputs a multiplexed signal; each of the multi-longitudinal mode lasers is configured to output multi-longitudinal mode signals having a periodically repeated frequency interval, wherein the period is Δfmode; the cyclic wavelength demultiplexer/multiplexer is configured to: perform filtering on the multi-longitudinal mode signals by using spectral filter windows corresponding to the input ends wherein the repetition period of the filter windows is Δfband and wherein centre wavelengths of the filter windows have a same spectral distance Δfch, to obtain N single frequency signals, each corresponding to the filter window of the respective input end of the cyclic wavelength demultiplexer/multiplexer; and the cyclic wavelength demultiplexer/multiplexer is further configured to: multiplex the filtered single frequency signals from the input ends into a multiplexed signal and output the multiplexed signal to the reflector by using the output end of the cyclic wavelength demultiplexer/multiplexer, wherein Δfmode is different from Δfband=N*Δfch, and Δfmode and Δfband are not in an integer-multiple relationship; the reflector is configured to: reflect a multiplexed signal of a preset portion of power in the multiplexed signal back to the cyclic wavelength demultiplexer/multiplexer, and output a remaining signal other than the preset portion of power in the multiplexed signal from the output end of the reflector; the cyclic wavelength demultiplexer/multiplexer is configured to: demultiplex, after receiving the multiplexed signal reflected by the reflector, the multiplexed signal reflected by the reflector to single frequency signals, separately determine those of the filter windows that correspond to the demultiplexed single frequency signals and those of the input ends of the cyclic wavelength demultiplexer/multiplexer that correspond to the determined filter windows; and feed back, by using the determined input ends, the demultiplexed single frequency signals to the multi-longitudinal mode lasers connected to the determined input ends; and the multi-longitudinal mode lasers are configured to: lock on wavelengths of the single frequency signals fed back by the cyclic wavelength demultiplexer/multiplexer, and generate and output signals whose wavelengths are the same as the wavelengths of the single frequency signals fed back by the cyclic wavelength demultiplexer/multiplexer. an output end of each of the multi-longitudinal mode lasers is connected to a respective one input end of the cyclic wavelength demultiplexer/multiplexer, an output end of the cyclic wavelength demultiplexer/multiplexer is connected to an input end of the reflector, and an output end of the reflector outputs a multiplexed signal; each of the multi-longitudinal mode lasers is configured to output multi-longitudinal mode signals having a periodically repeated frequency interval, wherein the period is Δfmode; the cyclic wavelength demultiplexer/multiplexer is configured to: perform filtering on the multi-longitudinal mode signals by using spectral filter windows corresponding to the input ends wherein the repetition period of the filter windows is Δfband and wherein centre wavelengths of the filter windows have a same spectral distance Δfch, to obtain N single frequency signals, each corresponding to the filter window of the respective input end of the cyclic wavelength demultiplexer/multiplexer; and the cyclic wavelength demultiplexer/multiplexer is further configured to: multiplex the filtered single frequency signals from the input ends into a multiplexed signal and output the multiplexed signal to the reflector by using the output end of the cyclic wavelength demultiplexer/multiplexer, wherein Δfmode is different from Δfband=N*Δfch, and Δfmode and Δfband are not in an integer-multiple relationship; the reflector is configured to: reflect a multiplexed signal of a preset portion of power in the multiplexed signal back to the cyclic wavelength demultiplexer/multiplexer, and output a remaining signal other than the preset portion of power in the multiplexed signal from the output end of the reflector; the cyclic wavelength demultiplexer/multiplexer is configured to: demultiplex, after receiving the multiplexed signal reflected by the reflector, the multiplexed signal reflected by the reflector to single frequency signals, separately determine those of the filter windows that correspond to the demultiplexed single frequency signals and those of the input ends of the cyclic wavelength demultiplexer/multiplexer that correspond to the determined filter windows; and feed back, by using the determined input ends, the demultiplexed single frequency signals to the multi-longitudinal mode lasers connected to the determined input ends; and the multi-longitudinal mode lasers are configured to: lock on wavelengths of the single frequency signals fed back by the cyclic wavelength demultiplexer/multiplexer, and generate and output signals whose wavelengths are the same as the wavelengths of the single frequency signals fed back by the cyclic wavelength demultiplexer/multiplexer.