The reaction force of a fast steering mirror (FSM) directly transmits to the FSM's base, leading to dynamic coupling interference to the optical platform and degradation of laser beam quality in an adaptive optics system. In this paper a $\Phi {320}\;{\rm mm}$Φ320mm aperture symmetrically arranged reactionless FSM actuated by piezoelectric actuators was proposed. The corresponding dynamic equation about reaction force was established by rotational equation of Newton's second law, and the equilibrium condition of reaction force compensation was deduced. Then, the finite element (FE) piezoelectric-coupling method was used to simulate dynamic characteristics, which shows that the elimination ratios of the reaction force are 99% at low frequency (at 50 Hz) and 94.1% at high frequency (at 120 Hz). The experimental results show that the first resonance frequency of the FSM is 159.82 Hz, the tilting angle is $\pm {1.3}^\prime$±1.3', the reaction force at high frequency (at 120 Hz) is 112.95 N, and the elimination ratio of reaction force is 90.14%. The simulation and corresponding test results indicate that the developed reactionless FSM significantly eliminates reaction force and therefore improve the FSM's stability and pointing accuracy.