Abstract: According to the feature of main body yarns and edge ones in 1×1 three-dimensional braiding with complex rectangular cross-section, a twice four-step braiding algorithm was described, which is realized through moving two groups of yarn one by one with the four-step braiding technique. A program to simulate the motion of yarn was worked out based on the algorithm by using the script language of Python and Tkinter, and the law of yarn motion was analyzed. The visualized models of the 3D braided preform were constructed by using visualization tool kit(VTK) and Beta-Splines, and man-machine interactive operation was permitted.