The invention provides a method and equipment for mounting large-diameter pipelines in a tube well and an application method of the equipment. The method includes: conveying a first pipeline into the tube well through oblique traction, stacking a second pipeline on the first pipeline in a caliber opposite manner, rotating the pipeline, and performing joint welding at the same time. The equipment comprises a rotary base mounted at the bottom of the tube well, an electric hoisting device mounted at the top of the tube well, a hoisting rope and rotary rings mounted in the middle of the pipelines and used for reducing friction force. The application method includes: mounting the rotary base and the electric hoisting device at corresponding positions, using the hoisting rope to guide the first pipeline into the tube well through oblique traction from a first layer, arranging the pipeline on the rotary base, guiding the second pipeline into the tube well through traction, stacking the second pipeline on the first pipeline, setting a welding gun, using the rotary base to allow the pipeline to rotate, and welding the joint of the two pipelines at the same time. The method and equipment has the advantages that the equipment is simple in structure, pipeline cutting is not needed, welding workload is reduced, and welding dead angle of the welding joint is avoided.