Rotary braiding is an efficient composite preform molding process， and the design of process parameters during braiding is crucial to the preform’s forming quality. Traditional models， due to their neglect of the interactions between yarns， result in significant discrepancies between the predicted and actual braiding angles. Based on the mechanism of fabric braiding， this paper divides the interlacing point behavior regions during the braiding process， defines the generation and consumption speeds of yarn interlacing points， and establishes a mathematical model of rotary braiding based on the conservation of interlacing points in the preform. The model achieves accurate predictions of the braiding angle and yarn width of the preform. Furthermore， the validity of the model is verified through braiding experiments using a variable cross-section mandrel. Experimental results show that the model effectively reflects the hysteresis phenomenon of braiding angles and yarn width changes during the variable cross-section braiding process and explores the impact of different speed ratios on the surface quality of the preform.