The invention relates to an insulating testing device for a 110kV three-phase common-box bus. The device comprises an insulating sleeve which is internally provided with a single-phase conductor for applying voltage to a three-phase bus. The insulating testing device further comprises a housing. One end of the housing is connected with a three-phase basin type insulator of the three-phase bus, andthe other end is connected with a single-phase basin type insulator of the insulating sleeve. The housing is provided with three translating structures in one-to-one correspondence with the bus. Thetranslating structure comprises a body and a moving contact. The body is connected with the housing. The moving contact is movably connected with the body. One end, which extends into the housing, ofa single-phase conductor, is provided with a circular trough. One end, which extends into the housing, of the three-phase bus, is provided with a through hole for accommodating the moving contact. Compared with the prior art, the insulating testing device realizes advantages of random switching of the three-phase bus between a testing phase and a grounding phase, one-step finishing of a testing process, no requirement for a complicated wiring phase change operation, and small occupation space.