The invention discloses a decoupling calculation method for damp synchronization measurement of a shaft series subsynchronous vibration machine of a turbo-generator set, referring to the technical field of damp measurement of shaft series machine of a turbo-generator set, which comprises the following steps of: connecting a generator to a power grid, calculating the characteristic value of subsynchronous vibration of the whole system, connecting the generator which is completely the same with the parameters of the previous generator to the power grid to operate in parallel, then calculating the characteristic value of subsynchronous vibration of the whole system, determining a common mode vibration or a different mode vibration only generated while operating the two generators in parallel, judging subsynchronous vibration and external system decoupling when the two generators are in different mode vibration, determining the conditions for the generation of different mode vibration, solving the steady-state operation point of the generator when the system is in different mode vibration, obtaining a pure electrical damp excluding mechanical damp, measuring the total damp of different mode vibration, and calculating mechanical damp. The decoupling calculation method for damp synchronization measurement of a shaft series subsynchronous vibration machine of a turbo-generator set provided by the invention solves the problems of the normally used method for obtaining the parameter of mechanical damp.