Recent experiments demonstrated that hierarchical double contraction nanotwins (DCTWs), i.e., 101 1 101 1 nanotwinned structures, can simultaneously improve the strength and ductility of Mg-Li alloys. With Mg as model material using atomistic simulations and interface defect analysis, we characterize structural characteristics of boundaries associated with DCTWs and investigate plastic deformation mechanisms of double contraction nanotwinned structures. It is revealed that the boundaries associated with DCTWs are composed of 101 1 coherent twin boundaries (CTBs), symmetrical tilt grain boundaries (STGBs), and asymmetrical tilt grain boundaries (ASTGBs). Under mechanical loadings, the corresponding grain boundary defects along STGBs and ASTGBs act as sources for nucleating and emitting basal and pyramidal dislocations, enhancing the plasticity. Meanwhile, the densely distributed CTBs in DCTWs act as strong barriers for dislocation motion, strengthening the material.