Abstract: In order to promote the application of steel-concrete composite beam in bridge structures and crane beams, the fatigue performance of composite beams has attracted researchers' attention. However, the basis for judging the fatigue failure mode of composite beams has not given. In the paper, stress amplitude index was defined based on the main experimental results and reasoning at home and abroad, which was the basis for judging the fatigue failure mode of composite beams. The fatigue tests of composite beams with different shear connection degrees and different details of steel beams were carried out so that the specific criteria for fatigue failure modes was obtained combined with the related parameters of steel beam details given by Standard for Design of Steel Structures (GB 50017-2017). It verified the fatigue failure modes of 13 test beams with different shear connection degrees. The results showed that it was feasible to judge the fatigue failure mode by stress amplitude index.