The invention discloses a method and system for calculating the influence of excitation additional difference adjustment on the synchronous torque of a synchronous generator. The method includes the following steps that: the Phillips-Heffron mathematical model of a single-machine infinite bus system is expanded, a difference adjustment model under additional difference adjustment is established; the oscillation frequency of the single-machine infinite bus system under no difference adjustment is solved according to the eigenvalue of the state equation set of the single-machine infinite bus system under no difference adjustment; the oscillation frequency of the single-machine infinite bus system under additional difference adjustment is solved according to the eigenvalue of the state equation set of the single-machine infinite bus system under additional difference adjustment; the synchronous torque coefficient KS of an excitation system under no additional difference adjustment ( a condition that Xc=0) is calculated according to the oscillation frequency of the single-machine infinite bus system under no difference adjustment; the value of the synchronous torque coefficient KS of the excitation system under additional difference adjustment (a condition that Xc is not equal to 0) is calculated according to the oscillation frequency of the single-machine infinite bus system underadditional difference adjustment; and the difference value of the synchronous torque coefficient KS of the excitation system under no additional difference adjustment (the condition that Xc is equalto 0) and the synchronous torque coefficient KS of the excitation system under additional difference adjustment (the condition that Xc is not equal to 0) is obtained by means of comparison.