Abstract: A large number of microcracks are initiated at the weld defects of steel bridge decks after aging, which induces the interaction and fusion behavior between near-source cracks. In order to study the interactive growth and fusion characteristics of multiple fatigue cracks in steel bridge decks and their influence on fatigue life, ABAQUS-FRANC3D interactive technology was used to simulate the dynamic evolution behavior of multiple fatigue cracks in steel bridge deck welds, analyze the variation law of Stress intensity factor between multiple cracks, and reveal the influence of multiple crack sensitive parameters on weld fatigue life, and then a dynamic characterization method for collinear cracks at welds in steel bridge decks was proposed based on ligament connectivity criteria. The numerical analysis results show that according to the variation characteristics of Stress intensity factor, the growth process of collinear double cracks can be divided into four stages: separation stage, interaction stage, fusion stage and combined growth stage; When two cracks contact each other to form a concave crack, the Stress intensity factor of the fusion point increases sharply, and the crack rapidly evolves from a concave crack to a convex crack, which is basically consistent with the fatigue test results; The interaction and fusion effects of multiple cracks in the weld seam significantly reduce its fatigue life, e.g., a double crack with an initial size of 4 mm shows a 31.6% decrease in fatigue life compared with a single crack; The simplified analysis method based on ligament connectivity criterion for multi crack interaction and fusion can effectively predict the fatigue life of steel bridge deck with multiple cracks.