The invention discloses a multi-motor joint vibration mode balancing method for a single support shafting steam turbine generator unit. At present, during on-site dynamic balancing of a single support shafting supercritical steam turbine generator unit, vibration information of two ends of rotors is not provided and shaft vibrations influence each other by coupling, so that a certain difficulty is brought into the on-site dynamic balancing of a shafting. According to the multi-rotor joint vibration mode balancing method, unbalanced types of the shafting are judged and calculated by calculating vibration vectors and vibration mode harmonic components and combining characteristics of a single supporting shafting structure according to shaft vibration magnitude values and shaft vibration phases of each rotor of the shafting at a critical rotating speed and a working rotating speed, and then a joint vibration mode of multiple rotors is identified; and a weight-increasing scheme of each plane is directly obtained according to a lag angle, mass-basis response coefficients and a first-time weight-increasing interval, and a group of increased weights are added to a related plane once. By the multi-motor joint vibration mode balancing method, the balancing efficiency and the accuracy of the single support shafting rotors are significantly increased.