The rib-to-diaphragm welded joints have complex stress and multiple cracking modes. Determining the dominant cracking mode of such structural detail is the premise of evaluating its fatigue performance. Taking the structural design parameters of a steel bridge deck and diaphragm openings as an example, the dominant cracking mode of the structural detail was studied by finite element model; how the crucial structural parameters will affect the stress amplitude of each cracking mode was discussed, as well as how the dominant cracking mode of the structural detail might shift. The research results show that the influence range of each cracking mode is mainly distributed within adjacent longitudinal ribs and diaphragms. Increasing the top plate thickness will reduce the stress amplitude of each cracking mode by a relatively small range (around 5%), and thus will not lead to a shift in dominant cracking mode. Changing diaphragm thickness and spacing will affect the stress amplitude of each cracking mode differently. When increasing the diaphragm thickness and spacing, the dominant cracking mode of the structural detail shifted from Crack-II to Crack-I.