The invention relates to an interconnected grid active power standby shared capacity calculating method and system. The method includes the following steps that: fast standby capacity sharing demand is calculated based on the operating state of a faulty sub-regional grid and the total supported capacity of non-faulty sub-regional grids; the faulty sub-region grid is supported based on the fast standby capacity sharing demand, and slow standby capacity sharing demand is calculated on the basis of the operating state of the faulty sub-region grid which has been supported; and a shared slow standby capacity allocation scheme is determined on the basis of the slow standby capacity sharing demand. According to the interconnected grid active power standby shared capacity calculating method and system of the invention, the fast standby capacity sharing demand of the sub-regional grids is calculated, so that the types and number of standby sharing devices can be judged, shared capacity is reasonably allocated in the non-faulty sub-regional grids with the standby redundant capacity of non-faulty sub-regions considered; and technical support can be provided for mutual active standby supportbetween interconnected grids, the reliability of system operation can be improved, and the realization of the large-scale optimal configuration of active power standby resources can be benefitted.