A bi-axial half-butterfly flexure hinge for an fast steering mirror （FSM） was presented to adapt high stability accuracy of beam-pointing control performance in laser weapon systems. According to the requirements of reciprocating movements and high bandwidth provided for the FSM, the solid model of the bi-axial half-butterfly flexure hinge was designed. By applying Castigliano’s displacement theorem, the numerical model was simplified and deduced. Furthermore, to quantify the numerical model, natural frequencies of the finite-element analysis and experiments were carried out, of which the results were compared with the analytic solutions. The experiment results show that the in-plane natural frequency is 165.29 Hz. The comparison shows that the error between numerical analytic and experimentation is 1.3%, and the error between FEA and experimentation is 3.2%. It is proven that the bi-axial half-butterfly flexure hinge is an appropriate structure as a guide mechanism for an FSM system.