ABSTRACT: In recent decades, it has been found that stiffness of flexible bodies can be increased under the circumstance of large overall motions. But the conventional hybrid-coordinate dynamical model can’t reflect this dynamic stiffening effect clearly enough. Since the slender blades of the floating offshore wind turbine usually work with high rotation-speed, the dynamic stiffening effect of flexible blades should be taken into consideration. In this paper, a higher-order coupling model is proposed and then incorporated into an in-house program, DARwind, to investigate dynamic stiffening characteristics of the blades and its effects on a spar-type floating offshore wind turbine. It is found that the additional stiffness of slender blades increases with the augment of the rotating speed in the higher-order coupling model. Furthermore, dynamic characteristics of the spar-type floating offshore wind turbine in the higher-order coupling model are different with those obtained by the traditional hybrid-coordinate dynamical model.