The invention relates to a forming machining device and a machining method of a straight-through type plastic inspection well. The forming machining device comprises a working table, extrusion molding equipment, a laser cutting and welding all-in-one system, a carrying robot with a manipulator and a control system, wherein at least one extrusion die is arranged on the extrusion molding equipment; an extrusion head is arranged on each extrusion die; and the laser cutting and welding all-in-one system comprises a frequency conversion laser device, a light path system for transmitting laser beams and a laser head for adjusting and collimating light spots of the laser beams. A main pipe and a branch pipe are molded by extrusion; the height size of the main pipe is the total height size of the straight-through type plastic inspection well; and the main pipe and the branch pipe are subjected to laser cutting according to the position of an intersecting line, and are welded. The forming machining device can realize automatic and flexible batch manufacturing of the straight-through type plastic inspection wells; the difficulty of a forming process and the requirement on forming equipment are reduced, the production investment is reduced, and the manufacturing cost is decreased; and the main pipes of a well chamber and a well seat are integrally machined, so that the mounting and construction time of the straight-through type plastic inspection well is saved, and the working efficiency is improved.