An audio encoding and decoding method and a related apparatus are provided. The audio encoding method may include: determining a coding mode of a current frame; when determining that the coding mode of the current frame is an anticorrelated signal coding mode, performing time-domain downmix processing on left and right channel signals in the current frame by using a time-domain downmix processing manner corresponding to the anticorrelated signal coding mode, to obtain a primary channel signal and a secondary channel signal, where the time-domain downmix processing manner corresponding to the anticorrelated signal coding mode is a time-domain downmix processing manner corresponding to an anticorrelated signal channel combination scheme, and the anticorrelated signal channel combination scheme is a channel combination scheme corresponding to a near out of phase signal; and encoding the obtained primary channel signal and secondary channel signal in the current frame.