A coherent transmitter, comprising a first signal modulation module, a first photodiode, PD, and a controlle, wherein the first signal modulation module, the first PD, and the controller are connected to each other, and the first signal modulation module comprises a first modulator, a second modulator, a first beam splitter, a phase shift adjustment unit, a first beam combiner, and a second beam combiner, wherein the first beam splitter is configured to split an input first optical signal to obtain a second optical signal and a third optical signal; the first modulator is configured to modulate the second optical signal to obtain a first modulated signal; the second modulator is configured to modulate the third optical signal to obtain a second modulated signal, wherein the first modulated signal and the second modulated signal have a same phase; the phase shift adjustment unit is configured to: adjust a phase of a first sub-signal in the first modulated signal, and adjust a phase of a fourth sub-signal in the phase-adjusted first sub-signal, wherein there is a first phase difference between the phase-adjusted first sub-signal and the first modulated signal, there is a second phase difference between the phase-adjusted fourth sub-signal and the first modulated signal, and the second phase difference is twice as large as the first phase difference; the first beam combiner is configured to combine a second sub-signal in the first modulated signal and the phase-adjusted fourth sub-signal to obtain a first combined signal; the second beam combiner is configured to combine a third sub-signal in the phase-adjusted first sub-signal and the second modulated signal to obtain a second combined signal; the first PD is configured to perform optical-to-electrical conversion on the first combined signal to obtain a first electrical signal; and the controller is configured to control, based on the first electrical signal, a voltage applied to the phase shift adjustment unit, so that the first phase difference falls within a preset range. the first beam splitter is configured to split an input first optical signal to obtain a second optical signal and a third optical signal; the first modulator is configured to modulate the second optical signal to obtain a first modulated signal; the second modulator is configured to modulate the third optical signal to obtain a second modulated signal, wherein the first modulated signal and the second modulated signal have a same phase; the phase shift adjustment unit is configured to: adjust a phase of a first sub-signal in the first modulated signal, and adjust a phase of a fourth sub-signal in the phase-adjusted first sub-signal, wherein there is a first phase difference between the phase-adjusted first sub-signal and the first modulated signal, there is a second phase difference between the phase-adjusted fourth sub-signal and the first modulated signal, and the second phase difference is twice as large as the first phase difference; the first beam combiner is configured to combine a second sub-signal in the first modulated signal and the phase-adjusted fourth sub-signal to obtain a first combined signal; the second beam combiner is configured to combine a third sub-signal in the phase-adjusted first sub-signal and the second modulated signal to obtain a second combined signal; the first PD is configured to perform optical-to-electrical conversion on the first combined signal to obtain a first electrical signal; and the controller is configured to control, based on the first electrical signal, a voltage applied to the phase shift adjustment unit, so that the first phase difference falls within a preset range.