The invention discloses a method and a system for calculating the influence of excitation additional difference adjustment on the dynamic damping of a synchronous generator, wherein the method comprises the following steps: a single-machine infinite bus system Philips-Heiferon mathematical model represented by K1, K2, K3, K4, K5 and K6 is expanded to establish a difference adjustment model considering additional difference adjustment which is represented by K1, K2, K3, K4, K'5, K'6. According to the eigenvalues of the state equations of the single-machine infinite bus system without error adjustment, the oscillation frequency of the single machine infinite bus system without error adjustment is solved; according to the eigenvalues of the state equations of the single machine infinite bus system with additional deviation adjustment, the oscillation frequency of the single machine infinite bus system with additional deviation adjustment is solved; and the damping torque coefficient KD ofthe excitation system without additional adjustment Xc=0 and the damping torque coefficient KD of the excitation system after additional adjustment Xc is not equal to 0 are calculated. The differencebetween the damping torque coefficient KD of the excitation system without additional adjustment Xc = 0 and the damping torque coefficient KD of the excitation system with additional adjustment Xc not equal to 0 is compared. The results show that the damping torque coefficient KD of the excitation system with additional adjustment Xc not equal to 0 is higher than that of the excitation system with additional adjustment Xc = 0.