The invention discloses a method and a device for quantifying flexibility margin of power compensation of a power distribution network. The method comprises the steps of determining a plurality of initial operation parameters of a target power distribution network in an operating state by a power flow algorithm when each power compensation device in the target power distribution network does not output power; determining a preset maximum allowable network loss value; inputting each initial operation parameter and the preset maximum allowable network loss value into a power calculation formularelated to the flexibility margin quantification of the power compensation of the power distribution network so as to obtain at least one power compensation strategy meeting the constraint conditionsof at least part of the operation parameters, wherein the power compensation strategy is an active power value and a reactive power value which need to be output by each power compensation device; anddetermining the flexibility margin value of the power compensation of the target power distribution network according to each active power value and each reactive power value, thereby realizing the quantification of the power compensation and adjustment capacity of the target power distribution network under the condition of meeting the network loss value requirement, and more effectively adjusting the power compensation of the target power distribution network.