The invention discloses a method for evaluating influence of ultrahigh-voltage access on stability of a provincial power grid. The method is used for evaluating the influence on the stability after ultrahigh voltage is accessed to the provincial power grid based on a large power grid real-time simulation system. A large power grid real-time simulation system model containing ultrahigh-voltage access is established and the model contains provincial power grid data, ultrahigh-voltage access data and national networking data. On the basis of the established data model, the influences of different parallel reactor capacities on the steady-state voltage and the transient overvoltage of a system are calculated and so reasonable compensation capacity is determined. Then, power grid load flow, transient stability and small-disturbance stability after ultrahigh voltage is accessed are analyzed. Finally, the overvoltage problem after the ultrahigh voltage is accessed is calculated. The method disclosed by the invention is applicable to evaluating the influence of ultrahigh-voltage access on the stability of the provincial power grid.