Disconnecting switch is the key primary equipment of power system. Temperature rise, which indicates the load capacity of power equipment, especially of enclosed GIS disconnecting switch, can be calculated to make it an important reference value in engineering. This paper has built an equivalent thermal-circuit model to calculate the temperature rise of 220kV three-phase enclosed SF6 disconnecting switch based on heat transfer theory. The formula of convective and radiative heat transfer between GIS conductor and tank has been given. The calculation method of steady thermal field under operating conditions considering ambient temperature, contact resistance and load current has been proposed. The comparison between FEM and calculated results shows that the proposed model can be used to acquire the hot-point temperature-rise of GIS disconnecting switch under normal operating conditions quickly and effectively, thus providing an important reference on dynamic capacity-increase of power equipment.