The invention belongs to the technical field of on-line stimulation for third-defense-line control measures of a power grid, and more specifically relates to a multi-round low-frequency load reductionsimulation calculation method for the power grid. The method specifically comprises a first round and a second round. The first round includes the steps of first, calculating a frequency curve by using transient stability calculation, obtaining high-precision frequency offset and the duration after the power grid breaks down, and matching with a low-frequency load reduction strategy to reach loadreduction requirements of the next round; second, calculating the load capacity should be shed according to the low-frequency load reduction strategy, configuring the load capable of being shed whenload shedding is performed, and preferentially removing the interruptible load; and third, calculating the power flow again, and entering the second round after the convergence. In the second round, the operation and calculation process in the first round is repeated. The method realizes modeling for three defense lines in an on-line calculation program of the power grid for the first time, achieves a function of multi-round low-frequency load reduction and a function that the three defense lines are stably controlled by an electric power system, and improves the calculation accuracy of the simulation program when the power grid breaks down.