A wind turbine blade generally has complex structures including several layers of composite materials with shear webs. It is essential but also inherently difficult to accurately and rapidly calculate the cross-sectional properties of a complex composite blade for the structural dynamics and aeroelasticity analysis of the blade. In this paper, a novel mathematical model for calculating the cross-sectional properties of composite blades has been developed by incorporating classical lamination theory (CLT) with extended Bredt-Batho shear flow theory (EBSFT). The mathematical model considers the shear web effects and warping effects of composite blades thus greatly improves the accuracy of torsional stiffness calculation compared with the results from direct use of 3D laminate theories. It also avoids complicated post-processing of force-displacement data from computationally expensive 3D finite-element analysis (FEA) thus considerably improves the computational efficiency. A Matlab program was developed to verify the accuracy and efficiency of the mathematical model and a series of benchmark calculation tests were undertaken. The results show that good agreement is achieved comparing with the data from experiment and FEA, and improved accuracy of torsional stiffness calculation due to consideration of the shear web effects is observed comparing with an existing cross-sectional analysis code PreComp. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.