The invention relates to a special tool for top block machining and top block machining assembling structure. An upper plane and a lower plane of a process board are correspondingly provided with an upper flat bottom groove and a lower flat bottom groove, a cylindrical pin hole penetrates through the upper flat bottom groove and the lower flat bottom groove, and a screw hole is axially formed in the upper end of a cylindrical positioning pin; the lower end of the cylindrical positioning pin is provided with a boss extending outwards, and the cylindrical positioning pin is matched with a cylindrical pin hole in the mounting state; the boss hooks the bottom surface of the lower flat bottom groove, and the upper portion of the cylindrical positioning pin penetrates out of an upper plane of aprocess plate; a square positioning pin is fixed in each of the upper flat bottom grooves in the two sides of the cylindrical positioning pin, the bottom surfaces of the square positioning pins and one side surfaces of the square positioning pins are positioning surfaces, and the upper parts of the square positioning pins are exposed out of the upper plane of the process plate and are not higher than the cylindrical positioning pin; the top block is placed on the process plate, and through grooves span over the square positioning pins and are in clearance fit with the square positioning pins;and the upper portion of the cylindrical positioning pin is coordinated with an ejector rod hole, and the cylindrical positioning pin is fastened with an ejector block screw. The special tool is simple, convenient and small in size, and capable of saving the time of clamping and tool setting, and is suitable for one-step machining of a plurality of top blocks with multiple specifications.