The invention belongs to the field of fast reflectors, and discloses a flexible support-based reaction-force-free fast reflector, which comprises a plane reflector assembly, a flexible support assembly, a sensor assembly, an actuator, a reaction plate and a base, the flexible support assembly comprises an upper flexible support and a lower flexible support, and the plane mirror assembly is connected with the base through the upper flexible support; the reaction plate is connected with the other side of the base through a lower flexible support; the number of the sensor assemblies and the number of the actuators are both four, the sensor assemblies and the actuators are evenly distributed on the plane mirror assembly in a staggered mode, one ends of the sensor assemblies are connected with the plane mirror assembly, the other ends of the sensor assemblies are connected with the base, one ends of the actuators are connected with the plane mirror assembly, and the other ends of the actuators are connected with the reaction plate. According to the fast reflecting mirror, the counter-acting force of the fast reflecting mirror is eliminated, the influence of the motion of the fast reflecting mirror on the base is greatly reduced, and the performance of the whole system is improved.