Deformation twins usually play an important role in plastic deformation and undoubtedly influence the mechanical property of hexagonal close-packed (hcp) metals. Understanding the interfacial features of twinning boundaries and their evolution during the plastic deformation is crucial to hcp materials design, processing and application. In this work, the interfacial characteristics of contraction twin in deformed magnesium alloy are investigated by means of electron back-scatter diffraction, transmission electron microscope and high-resolution TEM. The results show that the misorientation angle across the boundaries of contraction twin can deviate from the theoretical value , resulting in that the twinning planes of the twin crystal and the matrix crystal do not coincide any longer. Multiple dislocation activities within the contraction twin and interfacial defects located on the twinning plane are also characterized. According to these experimental features, a possible mechanism responsible for such deviation phenomenon is proposed. In addition, the similar deviation phenomenon is also observed in the - double twin system.