The invention discloses a primary forming welding process for butt welding of a dissimilar metal thick wall of a control rod driving mechanism. The process comprises the following steps of: 1) assembling and spot welding a sealing housing and a pressure container through part, wherein the sealing housing and the pressure container through part are connected to form a sealing housing assembly; and2) putting the spot-welded sealing housing assembly in a vacuum chamber, welding weld joints between the sealing housing and the pressure container through part by electron beams by means of a vacuumelectron beam welding machine till welding is finished. The process disclosed by the invention welds by using the vacuum electron beams of high energy density, so that primary forming welding for buttwelding of a nickel base alloy and an austenitic stainless steel dissimilar metal thick wall is achieved. Compared with original means, the weld joint quality is stable, the welding deformation is small, the utilization ratio of materials is high, and the production efficiency is high.