Abstract: The permeability of preform is an important material processing parameter. To build a model of estimating permeability is significant for understanding the wetting mechanism of fiber to resin, forecasting the permeability and optimizing the process and material parameters accurately. In this paper, a statistical model of preform's longitudinal permeability is built based on the analysis on the microstructure of the preform used in the LCM composites processing method. Research is done into the influence of the unidirectional preform's microstructure and its parameters on its longitudinal permeability, and it is verified in experiments finally.