Abstract: Based on the further research on the structure of modern multi-axial warp knitting composite perform and the analysis of its microstructure，a geometric model of multi-axial warp knitting composite perform with chopped fibers was developed by reasonable hypothesis.According to the model，the relation between geometric parameters and process variables was derived.And also，the theoretical fiber volume fraction formula of the perform was given out.Furthermore，Using different specification of perform as the samples，the theoretical value of fiber volume fraction was calculated efficiently by CAD software.Comparing the theoretical value with the experimental value，it indicated that the fiber volume fraction of the perform was in good agreement between theoretical and experimental value，which means that the model was practical.Finally，the factors leading to the deviations between theoretical value and experimental value were analyzed，they were mainly attributed to the ideal assumption of the perform，the resin-rich zones in the composites after the molding process and so on.