The invention provides a dual-dovetail type unmovable key bars assembling and welding process for a water turbine generator, which adopts a large-span equal division method to install key bars and welds the other key bars by lap welding the template within each large span. The key bar radius, radial skew angle, radius difference of adjacent bars and chord length difference of adjacent bars are adjusted up to standards by a support block C-shaped clip and other tools. The support block is welded by CO2 arc welding, the welding direction is from inner side to outer side, in order to whole controlling the welding deformation direction of the key bar chord length. The support block among each base ring plate are welded by skip welding, the radial bending welding deformation of the key bar is reduced. The invention has the advantages of simple and concise welding process, convenient assembly operation, high precision of key bar welding, high production efficiency and short assembly circle.