The invention provides a method for determining the degree of influence of a VFTO on a secondary device, and relates to the field of electromagnetic compatibility technology of secondary devices of transformer substations. The method provided by the invention comprises the following steps of: step 1: obtaining a physical model of a transformer substation, establishing a gas-insulated transformer substation circuit model by using electromagnetic transient simulation software according to the multi-conductor transmission line theory, in combination with the actual size of the transformer substation, and obtaining a very fast transient overvoltage waveform in the vicinity of the secondary device in the gas-insulated transformer substation circuit model; and step 2: according to the actual sizes of the secondary device to be detected in the transformer substation, a GIS conducting rod and a GIS shell, connecting the conducting rod, the shell and the secondary device, establishing a three-dimensional electromagnetic simulation model by using using electromagnetic transient simulation software, and inputting the very fast transient overvoltage waveform into the model to obtain the influence condition of the electromagnetic interference on the secondary device to be detected. The model of the method is simple, and the influence of the VFTO on the secondary sensitive equipment is visually and clearly reflected.