Abstract: Skeletal muscle is the driving tissue in human locomotion, and its macroscopic mechanical response depends on both multiscale structures and active-passive force-generating mechanisms. Previous studies have shown that mesoscopic structural parameters of skeletal muscle, such as fiber distribution pattern and fiber volume fraction, significantly affect its macroscopic mechanical response. However, the finite element methods based on continuum mechanics are insufficient to characterize these effects. Although the fully coupled FE² method can describe macro-micro mechanical response transfer, its high computational cost makes it difficult to apply directly to large-deformation dynamics simulations. In this study, a flexible multibody dynamic modeling method for skeletal muscle is developed based on the absolute nodal coordinate formulation (ANCF) and the multiscale finite element method, which accounts for both mesoscopic structural representation and computational efficiency. First, a macroscopic flexible multibody model composed of muscle fascicles and fascia is established, and the Mortar method is employed to construct non-matching interface constraints. A representative volume element (RVE) model of the muscle fascicle, consisting of muscle fibers and connective tissue, is then developed. The equivalent mechanical response of the muscle fascicle is obtained by introducing periodic boundary conditions derived from the macroscopic computation, and the macroscopic material parameters are calibrated accordingly. During the simulation of the macroscopic dynamic system, a parametric updating strategy based on the deformation gradient at representative points is introduced to enable online updating of macroscopic material parameters. On this basis, a solution procedure is established so as to combine the macroscopic dynamic response, microscopic homogenization computation, and online updating of macroscopic material parameters. The numerical simulations show that the proposed method can effectively characterize the effects of muscle fascicle microstructure and fascial force transmission on the macroscopic dynamic response of skeletal muscle, thereby providing a theoretical basis and an analytical tool for understanding the multiscale dynamics mechanism of muscle injury and further exploring the biomechanical mechanism of human motion.