In view of the common issue of composite porosity, the formation and influence mechanism of composite porosity are elucidated from the aspects of mechanical mixed nucleation theory and classic nucleation theory. The characterization of porosity morphology, distribution and size are conducted through specific test. The results show that: poor wettability of fibers, retention of vapor molecules and molecular volatilization generated during the crosslinking reaction are the main reasons of porosity formation. The lower cure pressures have a little effect on eliminating the porosity. With the increase of cure pressure, the porosity factor and porosity size decrease significantly. With the method of data fitting, the relations of porosity factor, porosity diameter and cure pressure are obtained, and the power function relations of three times are existed among them. This paper provides a theoretical and experimental support for optimizing the design of composites manufacturing process.