Abstract: Abnormal phenomenon was observed on the metallographic samples of grids spot-welded to guide thimbles when CJNF firstly used the thick guide thimbles (thickness 1.18 mm). Some tests focusing on the metallographic examinations were carried out. The factors affecting spot welding of grids to thick guide thimbles and the principle of nugget formation were analyzed. The analysis results set up good foundation for the future manufacture.