Abstract: 16Mn steel welded joints are tested to fail under low cycle fatigue,high cycle fatigue and combined cycle fatigue using an apparatus that is capable of providing interactive loading. A linear cumulative damage theory is used to estimate the fatigue life of the butt joints. The proportion of each of the two fatigue load components which cause the cumulative damage is calculated. The research indicate that high cycle fatigue loads and low cycle fatigue loads simply calculated as proportion would seriously underestimate the coupling between the two components of the fatigue load when we use Miner rule to calculate the fatigue cumulative damage of specimens. However,we can take into account of the coupling between the high and low cycle fatigue loads when we turn to the outsourcing envelope of the low cycle fatigue loads.