Abstract: A pneumatic liquid on-line automatic balancer of a rotor includes: a sealed balancing disk rotating with a balanced rotor and a pressure transfer device mated with the sealed balancing disk. At least two pairs of centrosymmetric chambers filled with appropriate balancing liquid are evenly distributed in the sealed balancing disk. The pressure transfer device controls compressed gas in and out of the chamber and makes the balancing liquid controllably flow in each pair of chambers to achieve dynamic balance of the rotor. A communicating channel of the balancing liquid is provided between each pair of the chambers. The communicating channel of the balancing liquid is one and a center of the communicating channel of the balancing liquid is communicated with the balancing channel pointed to a center of rotation. The balancing channel is communicated with an emptying channel opened to atmosphere.