The embodiment of the invention discloses a stereo encoding and decoding method, a coder-decoder and an encoding and decoding system, which belong to the technical field of multimedia and can encode and decode an embedded stereo signal. The stereo encoding method comprises the following steps: judging the sampling rate of a stereo signal and selecting a corresponding encoding mode; calculating the average value and the difference value to a left channel and a right channel of the stereo signal to respectively obtain a median signal and a sideband signal of the stereo signal; and respectively encoding the median signal and the sideband signal to form a code steam with an embedded structure. Relevant to the encoding process, the stereo decoding method comprises the following steps: decoding the code stream with the embedded structure by selecting different code rate layers, respectively calculating a median signal and a sideband signal, and obtaining the stereo signal by the synthesis of the median signal and the sideband signal. The embodiment can process a wideband stereo signal sampled by 16 kHz and an ultra wideband stereo signal sampled by 32 kHz by building a plurality of code rate layers.