The assembly clearance is one of the factors that affect the laser closure welding quality for international thermonuclear experimental reactor (ITER) correction coil case. An experimental study on relationship between the focused spot diameter and the butt joint gap margin in the defocusing amount of +5, +10, +16 mm respectively is performed in laser butt welding of 5 mm thick 316LN steel for nuclear fusion with hot wire filler under various butt gaps. The weld topography and cross section, metallographic phase, tensile strength, micro hardness test, electron microscope and composition of cross-section are analyzed after welding. The results show that in laser welding with hot wire filler the focused spot diameter can not be only equivalent to the butt joint gap, but also can be 0.1~0.2 mm less than the butt joint gap when the weld is of good shape and the higher tensile strength is obtained. When it is 0.3 mm less than the butt joint gap, bad weld beads are obtained and the tensile strength of the weld decreases.