The invention discloses an online optimization decision method and system for manual emergency load reduction and a storage medium. The method includes the following steps of: carrying out safety evaluation on the current operation state of a power grid to obtain a safety margin; for the situation that the safety margin does not meet the requirements of safe operation of the power grid, calculating a real-time manual emergency load reduction strategy meeting the safety constraint; for the situation that the safety margin meets the requirements of safe operation of the power grid, simulating the expected faults defended by an automatic safety device, and carrying out safety evaluation on the quasi-steady-state operation mode of the power grid after the expected faults; and for the situationthat the requirements of safe operation of the power grid are not met after the expected faults, calculating an expected fault manual emergency load reduction strategy meeting the safety constraint.According to the scheme of the invention, the risk that the power grid does not meet the requirements of safe and stable operation due to offline load reduction strategy mismatch or insufficient control amount of the automatic safety device can be eliminated, and the risk cost of manual emergency load reduction accidents can be reduced.