The invention discloses a method and device for determining the voltage reactive sensitivity of an automatic voltage control system. The method includes the steps that a power grid is divided into a target area network, a buffering network and an external network, and the state information of the power grid at the current time is collected; according to the state information, equivalence is carried out on the external network through the external network equivalence method reserving the buffering network, and the external network obtained after equivalence, the target area network and the buffering network form a new power grid; the generator node type in the new power grid is adjusted; the voltage reactive sensitivity between PQ nodes in the target area network after the generator node type is adjusted is determined according to a voltage reactive sensitivity matrix between the PQ nodes. The target area network and a key boundary node are taken as analysis objects, and the formed equivalence network can largely reduce external network information, so that the accuracy of the voltage reactive sensitivity is high, time is saved, and efficiency is improved.