The invention discloses a method and system for determining the main transformer ratio of a power grid based on reactive power optimization configuration, which belong to the technical field of powersystem simulation and calculation. Main transformers of which the power factors are smaller than a power factor preset value are selected in the target power grid as a plurality of target main transformers for reactive power optimization; reactive power flow directions of the plurality of target main transformers are monitored; the transformation ratios of the high-voltage sides of the plurality of target main transformers are adjusted upwards or downwards; and capacitance or reactance switching operation on the plurality of target main transformers is carried out, the adjusted data of the plurality of target main transformers are verified, whether the station voltage of the target main transformers meets an operation requirement is determined, a target main transformer ratio gear determination value and a power factor value are outputted, and the target power grid reactive power compensation configuration is determined to meet the requirement of power grid reactive power layering balance. The method is suitable for safety and stability evaluation of a large-scale power system, has good computability and practicability, and can be used as a main analysis tool for scheduling operation and planning design departments.