Due to the opening or closing operation of circuit breakers and disconnected switches in gas-insulated substations (GIS), a very strong transient wave process can be generated along the three-phase transmission system and transient electromagnetic field can be radiated from the junction between the GIS enclosure and the high-voltage cables. Based on a kind of hybrid method, the software TEMFGIS is provided to predict the radiated field in time domain. Its validity is verified by the measurement results