When the extreme geomagnetic event occurs, the extreme geomagnetically induced current (GIC) flowing through the transformer will produce a large amount of reactive power loss, causing grid voltage fluctuations. It is generally accepted that the increase in reactive power loss (GIC-Q) is proportional to the GIC flowing through the neutral point of the transformer, but under extreme conditions, the above proportional relationship will no longer apply. In this paper, a GIC-Q calculation method for extreme GIC is proposed, and a simulation model of the scaled transformer is established to analyze the reactive power characteristics of the scaled transformer when the extreme GIC is injected. The simulation results show that the reactive power loss of the transformer increases during extreme GIC injection, but is no longer proportional to the injection current. The simulation results further verify the accuracy of the proposed GIC-Q calculation method, and provide a reference for the evaluation of the reactive power of true power transformers under extreme GIC.