This paper studies automatic balancing of a flexible supported rotor with two disks by two ball balancers. Firstly, reduced dynamic descriptions of different components in the rotor are derived using a special method according to Lagrangian equation, based on which a general vibration model of the rotor system is then constructed. Secondly, equations governing frequency responses of the rotor are derived and working principles of the balls in the rotor are illustrated based on basic dynamic phenomena. Thirdly, an autonomous model of the rotor system is built with equations governing movement of the balls derived. Three equilibrium solutions are discovered whose dynamic characteristics are then studied using numerical bifurcation analysis method. Finally, locations of the balls at the steady state are observed using a high speed camera, and the capability of the two balancers for automatic balancing under different conditions is validated.