For the elastic deformation of cantilever type elastic structures in a flat spring landing gear, a new modal warp method was proposed to improve the calculation precision of the traditional modal method. Firstly, the description of a rotational component in infinitesimal deformation was deduced based on the finite element theory. Then the general modal method was summarized and how to introduce the rotational deformation into the dynamic equation in a modal coordinate was proposed. By which, the rotational deformation was introduced into the analysis of structure dynamics, so that the modal method could simulate the large bending and/or twisting deformation. Finally, the static simulation and experiments were performed and compared on the flat spring landing gear. The analysis results show that when the load mass is less than 100 kg, the relative errors are below 5% for both the linear modal method and modal warping method. However, when the load reaches to 180 kg(which represents the maximum realistic condition for landing), the error of the modal warp method remains within 10% and the error of linear modal has been more than 35%. Moreover, the time steps of both modal methods are 1 ms. As comparied with the traditional method, the modal warping method provides higher calculation and efficiency in the simulation of large rotational deformation.