Main shaft assembly positioning tool and system and welding method

The invention discloses a main shaft assembly positioning tool and system and a welding method. The main shaft assembly positioning tool comprises a rack, a first clamping assembly and two second clamping assemblies, the rack is provided with a base and a frame body, and the frame body is connected with the base; the two second clamping assemblies are used for clamping and fixing the two ends of the main shaft and driving the main shaft to rotate. The first clamping assemblies are provided with first positioning parts and second positioning parts, the first positioning parts are used for circumferentially positioning the weldment so that the weldment and the main shaft located between the two second clamping assemblies can be coaxially arranged, and the second positioning parts are used for axially positioning the disc-shaped weldment in the first direction. The main shaft assembly positioning tool can be suitable for assembly positioning of disc-shaped weldments with different outer diameter sizes and the ma