Abstract: The relationship between fatigue crack growth and self-emission magnetic signal for 20CrMo steel weld seam at the square wave load was studied by tension-tension fatigue experiment and metal magnetic memory test method.The characteristics of fatigue fracture were studied by SEM.The results show that with the increase of fatigue cycles,the self-emission magnetic signal of the weld seam increased,the fatigue crack growth rate firstly increases and then decreases before the crack reaches the weld.When the crack across the weld seam,with the increase of fatigue cycles,the self-emission magnetic signal of the weld seam begins to decrease,but the fatigue crack growth rate incrases rapidly.In different zones of the welded joint,the fatigue crack growth rates are different,the expansion rate for the heat-affected zone is faster,while in other regions the expansion rate is relatively slower.The fracture scanning analysis reveals that cleavage fatigue fracture occurs under the condition of square wave loads.