Abstract: Being lack of related theory up to now, the designers of 3-D preform are confronted with difficulty when calculating the structural parameters of 2.5-D preform. For this reason, this paper defined the concept of the similar 2.5-D preforms, and worked out the equations of structural parameters’s inner relationships of similar 2.5-D preforms by keeping their fiber volume fractions fixed. By means of the equations, new preform structural parameters could be calculated based on original preform structural parameters, and especially preform thickness could be predicted truly. The equations developed by the paper was verified by three experimental examples. The results show that the 2.5-D preform structural parameters equations possess practicability.