The invention discloses a laser downward butt weld method of a wide thick stainless steel plate. An I-shaped groove is adopted in a joint of the steel plate, interlocking portions are arranged on groove surfaces on the two faces, each interlocking portion comprises a ball head at the front end and a neck portion at the rear end, the ball heads are spherical, the neck portions are columnar, the diameter of the ball heads is larger than the diameter of the neck portions, the interlocking portions are alternatively arranged on the two sides of the groove, the strength of the welding portion is obviously improved, the repeated design experiment of the groove is avoided, time is saved, the round ball heads are adopted in the interlocking structures, appearing of cold cracks by stress concentration brought by tip corners after welding is avoided, and meanwhile the diameter of the ball heads is larger than that of the neck portions, so that the strength is further improved.