Side wrinkling may be produced during rotary-draw bending process of thin-walled rectangular 3A21 aluminum alloy tube if the process parameters are inappropriate. To study the side wrinkling behavior, a three-dimensional finite-element model of the process was developed based on ABAQUS/Explicit code and its reliability was validated by experiment. Then, the side wrinkling characteristics of the tube were investigated. The results show that the side wrinkling wave height in the inner edge of the tube is smaller than that in the out edge of the tube, and the side wrinkling wave height in the neutral layer of the tube is minimal. With the bending process, the side wrinkling wave goes through producing, developing and stabilizing three stages in turn. After bending angle reached 30°, the side wrinkling wave height keeps almost unchanged. Furthermore, the number of the half side wrinkling wave increases evenly.