Using AMD system, a method for active control of long span cable-stayed bridges under seismic loads is presented. The motion equation order of bridge is reduced by modal technique, and the classical linear optimal control theory is employed. The present method has been used in the active control of seismic response of the Second Nanjing Yangtze River Bridge subjected to EL-Centro and man-made earthquake excitations. The controlled modes number and actuator configurations of bridge are discussed in details. Subsequently, the parameters of AMD system of long-span cable-stayed bridge are designed and optimized. The influences of mass and spring stiffness on the performance index and the maximum actor effect are analyzed. The basic parameters of AMD adapting to the Second Nanjing Yangtze River Bridge are recommended.