Abstract: Based on the iso-strain and interfacial continuities conditions, a theoretical method is proposed to predict the elastic properties of triaxially braided composites concerning the variation of the braid yarn’s undulation period along its transverse direction in particular areas. The theoretical method was verified by comparing the analytical results with the test results. Parametric studies are conducted in the same fiber volume fraction. It is found that: 1) Increasing the braid angle leads to the decreasing of the longitudinal modulus and the increasing of the transverse modulus. 2) The in-plane shear modulus peaks when the braid angle approaches 50°; Poisson’s ratio peaks when the braid angle approaches 35°. 3) Increasing the axial yarn size relative to braid yarn size leads to the increasing of the longitudinal modulus, the decreasing of the transverse modulus, the in-plane shear modulus and Poisson’s ratio.