Abstract: The microst ructure of a four-step 3D braiding preform with 1 ×1 pattern and complex rectangular crosssection was studied by means of the cont rol volume method. The complex rectangular configuration refers to the geomet rical shape jointed by two or more rectangles together. According to the formation rule of the braiding process ,the paths of the carriers are t raced , particularly in the joint region where two rectangles join together. In addition tothe microst ructures of the braiding preform in the interior , surface and corner of a simple rectangle , there are differentmicrost ructures in the joint region of the interior , surface and corner of the complex rectangle. Corresponding to thespecial microst ructure in the interior , surface and corner in the joint region , three unit cell models were developed.The result shows that , in the joint region , the geomet ry of the unit cell in the interior , corner and surface is of acube , a hepta-prism and a penta-prism , respectively. In addition , the braiding pattern and braiding angle of the unitcells in the joint region differ significantly f rom those of the simple rectangle of a 3D braiding preform.