The present invention relates to the field of signal processing, and discloses code modulation and demodulation methods, apparatuses, and a system, so as to reduce an envelope fluctuation degree of a signal and improve system performance. A specific implementation method includes: receiving, by a sending apparatus, N channels of bit signals, and performing serial-to-parallel conversion on any channel of bit signal of the N channels of bit signals, to generate two channels of first bit signals; performing delay control on the two channels of first bit signals, to generate two channels of code bit signals; performing QAM mapping on the two channels of code bit signals and other N−1 channels of bit signals, to generate a channel of quadrature electrical signal and a channel of in-phase electrical signal; modulating the quadrature electrical signal and the in-phase electrical signal, to obtain a transmit signal.