The invention relates to a GIS reserved interval, an external line expansion method and GIS equipment. The GIS reserved interval is arranged in the GIS equipment and comprises a first bus, a second bus, a first isolating switch, a second isolating switch, an expansion isolating switch and an expansion interface; the first bus is connected to one end of the first isolating switch; the second bus is connected to one end of the second isolating switch; the other ends of the first isolating switch and the second isolating switch are connected through a connection conductor; one end of the expansion isolating switch is connected to the connection conductor and the other end is connected to the expansion interface; and the seven parts are arranged in seven independent air chambers respectively. During construction, air pressure in the air chamber in which the expansion isolating switch is located is reduced to a set value; the air pressure of the expansion interface in the air chamber is controlled to be barometric pressure; and then subsequent operation of expanding an external line is carried out. By the method, a widespread power failure can be avoided; and the overall safety, reliability and convenience of the expansion work are ensured.