The invention provides a photovoltaic power plant station-level island prevention protection method and system. The method comprises the steps of disconnecting all inverters in the photovoltaic powerplant when the grid-connected point bus frequency of the photovoltaic power plant exceeds an island prevention protection range; when the grid-connected point bus frequency of the photovoltaic power plant does not exceed a preset island prevention protection range, determining island formation possibility in the photovoltaic power plant by comparing variable quantity and threshold values of the grid-connected point bus frequency of adjacent two frequency periods; and when it is determined that island formation possibility exists, comparing grid-connected point bus frequency and power frequency, and determining a corresponding preset control strategy according to the comparison result to adjust reactive power output by all the inverters until the grid-connected point bus frequency exceeds the island prevention protection range. Compared with the prior art, the photovoltaic power plant station-level island prevention protection method and system provided by the invention is applicable toisland prevention protection with multiple inverters without causing a monitoring blind area and with high reliability.