The invention relates to the field of centrifugal compressors, in particular to a built-in bidirectional variable cross-section exhaust volute chamber structure of a centrifugal compressor. The centrifugal compressor is divided into two sections, the first section of the centrifugal compressor is provided with a built-in exhaust volute chamber, and the second section of the centrifugal compressor is provided with a conventional overhead exhaust volute chamber. A partition plate is arranged between the two sections of the compressor. A first deflector, a second deflector and a third deflector are arranged above the partition plate. The first deflector, the second deflector, the third deflector and the partition plate together form the built-in exhaust volute chamber. The first deflector is a spiral arc plate, and the diameter of the first deflector is gradually decreased relative to the center of the compressor. The first deflector and the second deflector enable the built-in exhaust volute chamber to become a variable cross-section runner. According to the built-in bidirectional variable cross-section exhaust volute chamber structure of the centrifugal compressor, the original two sections with great structure size difference of the centrifugal compressor can be contained in a shell body, the size of the compressor is decreased, and meanwhile the bearing capacity of the compressor is improved.