Combined with ultrasonic phased array， metallographic observation and mechanical test analysis technology， the influence of welding gap caused by different milling angles on the welding quality and mechanical properties of friction stir welding seam on tank dome cover was studied. The results show that when the welding gap caused by the difference of milling angle is greater than 1.30 mm， there are void volume defects in the weld. The larger the welding gap is， the more serious the defect is. The existence of welding gap reduces the density of the weld and the elongation of the weld. When the welding gap is greater than 1.30 mm， the tensile strength of the weld decreases greatly.