The invention provides a thermal shrinkage molding method of a C-shaped beam of a variable cross-section rectangular pipe. The method comprises the following working procedures: (1) carrying out equivalent drawing-down, namely, heating the two ends of a seamless round tube by an intermediate frequency furnace, and then making the two ends of the round tube into conical tubes, wherein the small end is arranged outside; (2) carrying out variable drawing-down, namely, heating the conical tubes at the two ends of the round tube by the intermediate frequency furnace based on the working procedure (1), and making the conical tube at each end into two sections of continuous conical tubes with different taper angles; (3) processing the middle part by square pushing, namely, enabling the circular tube in the middle part to be rolled into a square tube in a forced way by a square pushing machine; (4) pressing the two ends to be square, namely, pressing the two sections of conical tubes with different taper angles at the two ends into two sections of tubes which have different taper angles and rectangular cross sections; and (5) carrying out whole bend molding, namely, pressing the whole tube into a C-shaped beam body. The molding method is fewer in working procedures, high in production efficiency and high in product quality stability.