The cross-sectional distortion usually appears during rotary-draw bending process of thin-walled rectangular tube with small bending radius. To study the cross-sectional distortion of the tube, 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 cross-sectional distortion behaviors of the tube were investigated. The results show that a zone of larger circumferential stress appears on the tube when bending angle reaches 30-. And in the larger circumferential stress zone, the sagging phenomenon is produced obviously. The maximum cross-sectional distortion is located in the larger circumferential stress zone and the angle between the plane of maximum cross-sectional distortion and the bending reference plane is about 50-. The position of the maximum cross-sectional distortion keeps almost unchanged with the variation of the clearances between dies and tube.