In this paper, we propose the novel gait trajectory adaptation method applicable to an exoskeleton for strength augmentation. The proposed method consists of the online positive feedback trajectory adaptation method and the swing phase duration adaptation method. The swing test results of an exoskeleton prototype showed that the proposed method is more adaptable to the change of the trajectory amplitude than the general trajectory control method mainly used for rehabilitation and it can adjust the swing phase duration as the walking speed changes. This method can be utilized to design the fast walking control of an exoskeleton in a swing phase.