A stereo encoder, comprising: a preprocessing unit, configured to perform time domain preprocessing on a left channel time domain signal and a right channel time domain signal that are of a current frame of a stereo audio signal, to obtain a preprocessed left channel time domain signal and a preprocessed right channel time domain signal that are of the current frame; a delay alignment processing unit, configured to perform delay alignment processing on the preprocessed left channel time domain signal and the preprocessed right channel time domain signal that are of the current frame, to obtain the left channel time domain signal obtained after delay alignment and the right channel time domain signal obtained after delay alignment that are of the current frame; a solution determining unit, configured to determine a channel combination solution of the current frame based on the left channel time domain signal obtained after delay alignment and the right channel time domain signal obtained after delay alignment that are of the current frame; a factor obtaining unit, configured to obtain a quantized channel combination ratio factor of the current frame and an encoding index of the quantized channel combination ratio factor based on the determined channel combination solution of the current frame, and the left channel time domain signal obtained after delay alignment and the right channel time domain signal obtained after delay alignment that are of the current frame; a mode determining unit, configured to determine an encoding mode of the current frame based on the determined channel combination solution of the current frame; a signal obtaining unit, configured to downmix, based on the encoding mode of the current frame and the quantized channel combination ratio factor of the current frame, the left channel time domain signal obtained after delay alignment and the right channel time domain signal obtained after delay alignment that are of the current frame, to obtain a primary channel signal and a secondary channel signal of the current frame; and an encoding unit, configured to encode the primary channel signal and the secondary channel signal of the current frame. a preprocessing unit, configured to perform time domain preprocessing on a left channel time domain signal and a right channel time domain signal that are of a current frame of a stereo audio signal, to obtain a preprocessed left channel time domain signal and a preprocessed right channel time domain signal that are of the current frame; a delay alignment processing unit, configured to perform delay alignment processing on the preprocessed left channel time domain signal and the preprocessed right channel time domain signal that are of the current frame, to obtain the left channel time domain signal obtained after delay alignment and the right channel time domain signal obtained after delay alignment that are of the current frame; a solution determining unit, configured to determine a channel combination solution of the current frame based on the left channel time domain signal obtained after delay alignment and the right channel time domain signal obtained after delay alignment that are of the current frame; a factor obtaining unit, configured to obtain a quantized channel combination ratio factor of the current frame and an encoding index of the quantized channel combination ratio factor based on the determined channel combination solution of the current frame, and the left channel time domain signal obtained after delay alignment and the right channel time domain signal obtained after delay alignment that are of the current frame; a mode determining unit, configured to determine an encoding mode of the current frame based on the determined channel combination solution of the current frame; a signal obtaining unit, configured to downmix, based on the encoding mode of the current frame and the quantized channel combination ratio factor of the current frame, the left channel time domain signal obtained after delay alignment and the right channel time domain signal obtained after delay alignment that are of the current frame, to obtain a primary channel signal and a secondary channel signal of the current frame; and an encoding unit, configured to encode the primary channel signal and the secondary channel signal of the current frame.