In order to expand the applied range of diode laser in laser processing field, and to apply diode laser to the thick plate welding of metal materials, the fiber coupled diode laser of LDF4000-40 that developed by the Laserline company is adopted to investigate the welding performance of the thick SUS304 austenitic stainless steel. Experimental results show that the keyhole can be formed, proving the ability of strong penetration. Compared to the fiber laser welding under the same welding conditions, the weld penetration depth decreases and bead width increases slightly; weld forming and welding process stability is better, and spatter is significantly smaller. The results confirm that the fiber coupled diode laser can be used for the thick plate welding of metal materials.