The present invention relates to the technical field of a car stereo audio expanding method. The method performs delay processing on sound channel signals in a loudspeaker twice to counteract signals reflected by door glass, such that the sound field performs better. The sound field processing method of the present invention can enable the sound field of sounds heard by the driver to be centered and the diffusion of both sides to be symmetric. Also the sound field can be expanded to the outside of the vehicle, resolving the problem that the car audio, in a car environment, has a sound field weighted toward one side, has a narrow sound field width, and has poor hearing effects due to the confined sound field. The invention enables sound heard to be authentic rather than empty.