The invention provides a swelling pressure composite forming method for a large-section-difference special-shaped section pipe fitting and relates to a forming method for a large-section-difference special-shaped section pipe-shaped part. The swelling pressure composite forming method comprises the main steps that 1, the perimeters and the equivalent diameters of a plurality of cross sections on the to-be-formed large-section-difference special-shaped section pipe fitting are determined, and the structure of a swelling pressure composite forming mold and the inner cavity size and initial position of a side extrusion mold are determined; 2, an equal-diameter circular-section pipe blank with a certain length is cut and put into the swelling pressure composite forming mold; 3, the equal-diameter circular-section pipe blank is filled with a pressure medium with a certain pressure through a pressure medium inlet, and a variable-diameter circular-section pipe blank is obtained through swelling; 4, the side extrusion mold is driven to extrude the variable-diameter circular-section pipe blank, and the large-section-difference special-shaped section pipe fitting is obtained; and 5, the pressure is released, and after the swelling pressure composite forming mold is opened, the formed pipe fitting is taken out. The swelling pressure composite forming method is used for forming large-section-difference special-shaped section pipe fittings.