Rotation is one of the critical motion forms in morphing wings. A rotation-locking joint can significantly improve aircraft stability and adaptability across different environments. However, designing a joint that combines stepless locking, high compact, energy efficiency and lightweight remains a considerable challenge. This paper proposes a novel flexible active locking mechanism for rotational motion. Comprising a piezoelectric actuator, a flexible lever mechanism, and a flexible bridge mechanism, it allows locking at any position. A nonlinear model of the mechanism is developed using the compliance matrix method to predict its amplified displacement output in response to input forces. The model’s accuracy is validated through Finite Element Analysis (FEA). Finally, a locking force experiment further demonstrates the feasibility of the locking mechanism. This compact locking system holds significant potential for aerospace application.