The utility model provides a test loop comprising a first isolating switch device, a second isolating switch device, a first isolating pipe sleeve, a second isolating pipe sleeve, as well as a branch bus, wherein one end of the first isolating switch device is connected with one end of the second isolating switch device through a pipeline, while the other end of the first isolating switch device is connected with one end of the first isolating pipe sleeve through a pipeline; the other end of the second isolating switch device is connected with one end of the second isolating pipe sleeve through a pipeline; one end of the branch bus is connected on the pipeline between the first isolating switch device and the first isolating pipe sleeve, while the other end is suspended; and different GIS (gas insulated substation) connection modes in a GIS are imitated by changing the length of the branch bus, so that the VFTO (very fast transient overvoltage) wave forms occurring at the disconnection port of an isolating switch and a GIS pipeline under different GIS connection modes in the GIS are imitated.