A new method to increase the applied bandwidth of piezoelectric Fast Steering Mirror（FSM）is proposed. Firstly, according to the Lagrange Equation, dynamic equation for simplified model of this FSM was built based on radial fasten thin plane. Then, the final theoretical expression of the steering resonant frequency was deduced for purpose of predicting the dynamical performance of the FSM. On the base of the theoretical results, we fabricated this structure of thin plane, added it to the original FSM and took the frequency-response and angle-stroke experiments. Both the experimental results and theoretical analysis results proves that the radial fasten thin plane is useful in increasing the applied bandwidth of FSM.