The rapid increase of transformers cluster reactive power loss resulted from geomagnetic disturbance (GMD) threats the system voltage stability fundamentally. Whether the voltage is close to collapse or not will help operator in time to take scheduling measures to avoid a more serious accident. Based on the analysis of the fault mechanism of the power system due to GMD, the transformer cluster reactive loss is made reasonably equivalent and the method and index of voltage stability judgment based on catastrophic theory is proposed according to the same features of relation between voltage and power system parameter and the canonical form of swallowtail catastrophe. The simulation on the 10-machine New-England power system with 39 nodes shows the effectiveness of the method.