After the third generator set of Jiangya Water Power Station was put into operation for one year, its stability got worse. So an overhaul was conducted. Through analysis on stability test results it is found that the extraordinary sway and vibration was caused by unbalance of electricity and unbalanced weight-distribution in the set. So weight distribution of the set was adjusted and slight deviation of barycenter of rotor was handled;the generator set is put into operation again and works in perfect order.