The present invention relates to a modulation circuit of a digital transmitter, a digital transmitter, and a signal modulation method. The modulation circuit includes: a first synchronizing circuit and a digital modulator, where the first synchronizing circuit separately perform phase delay on a first local-frequency signal or a second local-frequency signal to obtain corresponding delay signals, and perform phase adjustment on a digital baseband signal by using the delay signals, to generate a first adjusted signal and a second adjusted signal; and the digital modulator modulates the first adjusted signal by using the first local-frequency signal, to generate a first radio-frequency signal, and modulates the second adjusted signal by using the second local-frequency signal, to generate a second radio-frequency signal.