The invention provides a twisting beam back shaft, which can guarantee the service life of the back shaft and meanwhile saves space and reduces the weight of the complete vehicle. The twisting beam back shaft of the invention comprises a beam and a pipe fitting-formed suspension trailing arm which connects to the left and right ends of the beam; in particular, the top surface of the beam is horizontal, and the cross sectional area of the beam increases gradually from the middle part to the two sides. By changing the distribution of the cross sectional area of the beam, the twisting beam back shaft of the invention can effectively avoid stress concentration at the welding seam where the back beam and the trailing arm are overlapped, and thus prolongs the service life of the back shaft assembly; the top surface of the beam is horizontal unlike the upward convex traditional beams, which can largely save the required space for arrangement, improve the space utilization of the complete vehicle, reduce the weight of the complete vehicle and fuel consumption.