Abstract: This paper systematically studies the full-life cycle fatigue behavior of the welded joints of Q355 low-carbon steel under cyclic loading. Through experimental analyses in aspects such as the microstructure, crack initiation, and propagation behavior, as well as the temperature evolution law, the full-life fatigue evolution mechanism and the energy evolution law of the welded joints of Q355 steel under cyclic loading are revealed. During the fatigue process, the surface temperature of the material can be divided into the initial rapid temperature rise stage, the stable temperature rise stage, the rapid temperature rise stage, and the sudden temperature drop stage. The tensile strength of the welded joint is 3.17% lower than that of the base material, but the yield strength is significantly increased (by 9.60%), and the ductility remains basically unchanged. The fatigue limit of the welded joint of Q355 is 390 MPa. The crack propagation process conforms to the law of the Paris formula, and the crack propagation rate increases with the increase of the stress intensity factor.