This study presents the design of a flexible mechanism with a large deflection angle for piezoelectric-driven fast-reflecting mirrors, to address the common issue of small deflection range. First, a study was conducted on the correlation among the nested hierarchy, configuration, natural frequency, and amplification factor of the flexible mechanism. Accordingly, a preliminary plan was developed for the design of a three-stage hybrid configuration. The mechanism was discretized into flexible hinges, rigid bodies, and concentrated masses as basic units. Subsequently, a general dynamic stiffness model was constructed for the flexible mechanism using the matrix displacement method. This model establishes a mapping relationship between the structural parameters of the flexible mechanism and the deflection angle of the fast-reflecting mirror. On this basis, a modal analysis of the flexible mechanism was performed,whereby the key dimensional parameters of the fast-reflecting mirror's flexible mechanism were optimized.Compared to similar research conducted domestically and internationally, this configuration achieves a mechanical deflection angle greater than 100 mrad by ensuring miniaturization and a higher first-order natural frequency.