Inertial positioning platforms driven by piezoelectric actuators have been employed in many precision applications. However, challenges including unstable pose keeping and intractable displacement backward still exist. In this study, a locking device is designed and embedded in a piezoelectric inertial rotary platform to achieve the functions of power- off locking and 2-degree of freedom (DOF) backward motion suppression. The vertical output displacement of the locking end is converted from the horizontal elongation of the lead zirconate titanate (PZT) stack by the flexible hinge mechanisms of the locking device, which is used to lock the rotor of the rotary platform. Theoretical and simulation analyses of the locking mechanism are performed and its dominant parameters are optimized. A prototype is fabricated and a series of experiments are performed. Compared with the inertial rotary platform without the locking device, the locking ability is increased by 4.55 times and the backward ratio is reduced from over 60% to below 12%. The application of locking device not only improves the pose keeping ability of the inertial positioning platform, but also improves its displacement output characteristics through backward motion suppression.