Abstract: In the braiding procedure, all of yarns are interwoven by the intermittent movement of carriers. The braiding yarn is tended to move along the direction of the carrier movement. In this paper, a method of least square approximation was introduced to predict the trend line of carrier movement. Then the yarn paths were analyzed systematically. Finally, two kinds of unit cells, the interior cell and the surface cell, were established. They are oriented in the same reference frame as the preform, which is quite suitable for the analysis of the mechanical properties.