The invention relates to an automatic voltage control (AVC) system of a power station and a reactive real-time adjustment quantity calculation method of the AVC system. A complicated relation between parameters such as a reactive power and a sub voltage in the power station is simplified according to a series of characteristic values, a model of changing quantity generated parameters such as a high/low-side bus voltage and an output reactive power of the power station is built after reactive output of the power station is increased, an reactive compensation power of a high-voltage side bus is increased or the position of a first-shift transformer tap switch is raised or descended; during in-site running of the AVC system of the power station, the model can be used as an internal algorithm and is introduced to the AVC system, the AVC system is assisted to rapidly and accurately make a reactive compensation switching strategy, so that a new AVC system of the power station is generated; and by the new AVC system, the rapid adjustment response time requirement of voltage and reactive power can be met, so that the voltage and the reactive power of the power station are maintained within a qualified range at any time, and the electric energy quality output from the power station is improved.