The invention discloses a forming method for improving wall thickness distribution of a superplastic forming special-shaped cylinder body through predeformation. The method comprises the following steps of (1), carrying out designing of an aluminum alloy special-shaped cylinder body blank; (2), carrying out designing of a superplastic forming upper mold and a superplastic forming lower mold of thealuminum alloy special-shaped cylinder body; (3), carrying out designing of an superplastic forming core mold of the aluminum alloy special-shaped cylinder body; (4), carrying out reverse superplastic forming on the aluminum alloy special-shaped cylinder body; and (5), carrying out forward superplastic forming on the aluminum alloy special-shaped cylinder body. Reverse superplastic forming is carried out and then forward superplastic forming is carried out so as to prepare the aluminum alloy special-shaped cylinder body, thus the problem that the size precision and the molded surface precision are difficult to control in conventional cold stamping or mechanical machining is solved, and the surface quality of a part is improved; and aluminum alloy special-shaped cylinder body reverse superplastic forming is adopted to carry out predeformation on the aluminum alloy special-shaped cylinder body blank, then aluminum alloy special-shaped cylinder body forward superplastic forming is adopted so as to carry out final forming, thus the defect that the wall thickness distribution of the aluminum alloy special-shaped cylinder body only subjected to forward superplastic forming is not uniform is overcome, and the requirements for size precision, molded surface precision and surface quality of the part are guaranteed.