To solve the on-site non-outage extension and voltage withstand test of 110 kV three-phase common enclosure gas insulated switchgear (GIS), a new type of 110 kV three-phase common enclosure double-fracture disconnecting switch (DDS) is presented. First, the structural characteristics and construction scheme of the three-phase common enclosure DDS are introduced, and its feasibility to realize the extension and voltage withstand test under non-power outage of 110 kV GIS busbar is demonstrated, and the initial design scheme of three-phase common enclosure DDS is given accordingly. Then, the designed three-phase common enclosure DDS is verified by the calculation of internal insulation, long-term temperature rise and dynamic stability based on the simulation of finite element electric field, temperature flow field and mechanical structure respectively. The results show that the designed three-phase common enclosure DDS has sufficient electrical insulation strength, margins of maximum temperature rise and structural strength. The designed three-phase common enclosure DDS has passed the full set of type tests required by national standards, which meets the demand of applying to practical projects.