A stereo encoding method and apparatus, and a stereo decoding method and apparatus are disclosed. The stereo encoding method includes: performing downmix processing on a left channel signal of a current frame and a right channel signal of the current frame, to obtain a primary channel signal of the current frame and a secondary channel signal of the current frame; and when determining that a frame structure similarity value falls within a frame structure similarity interval, performing differential encoding on a pitch period of the secondary channel signal by using an estimated pitch period value of the primary channel signal, to obtain a pitch period index value of the secondary channel signal, where the pitch period index value of the secondary channel signal is used to generate a to-be-sent stereo encoded bitstream.