Abstract: Two main problems must be solved in order to obtain a reliable slab laser gain medium module with large size and low wave-front distortion during high power working condition. The first problem is the formation of a large connection area with low thermal resistance and free voids. The second problem is how to get a low stress connection with low transmission wave-front distortion. The indium metal was chosen as a welding material for its outstanding ductile property and its optimal thickness was found through experiments. A reliable connection area greater than 40 cm2 has been established between slab laser gain medium and cooling unit by improved welding technique. The repetitiveness of this improved craft with voids ratio less than 0.5% and voids area less than 1 mm2 is over 90%. The ones whose static transmission wave-front distortion is less than 1 m of slab laser gain medium with the size of 150.2 mm30 mm2.5 mm is over 80%, while those with the static transmission wave-front distortion less than 1.5 m is nearly 100%. A 4000 W-output Nd: YAG slab laser has been achieved based on improved packaging technology.