A large number of ship-borne mast failure accidents show that fatigue damage is the biggest hidden danger of mast structure. This paper analyzed the major failure factors and prevention method of girth weld fracture of a mast based on finite element method and test method. The results show that the structure where stiffeners are welded to the base of the mast retain high levels of welding residual stress, and therefore, reduce fatigue resistance of the mast. Furthermore, multi-directional wind loading can lead to the stress exceeding the fatigue strength in the girth weld area of the mast which promote initiation of fatigue crack. The effect of the thickness of the mast on the stress level of the mast is analyzed, and the results show that increasing the thickness of the mast can effectively reduce the stress level in the girth weld area, that is, the stiffness of the mast structure increases. The finite element analysis is validated by comparing the results of finite element calculation results and tests.