The mainline electric oil pump assembly relates to the field of assemblies for pumping oil in main oil pipelines. The aim of the present invention is to create a mainline electric oil pump assembly with improved technical and economic characteristics, reduced noise and vibrations, and increased reliability, service life and performance. The main distinguishing features of the proposed assembly which contribute to the achievement of the aforementioned aim are: the non-splined, zero-backlash seating of the impeller on a smooth shaft, the use of roller bearings seated in a non-splined, zero-backlash fashion on the smooth shaft, the use of mechanical seals seated in a non-splined, zero-backlash fashion on the smooth shaft, the use of self-correcting adjustable spherical supports under the supporting feet of the pump, and the boring of the seating surfaces of the pump housing for the rotor in a single set-up operation.