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The fatigue performance of dissimilar metal welded components has an important influence on the service reliability of spacecraft. The fatigue life and fatigue fracture mechanism of dissimilar metal welded joints of QCr0.8 copper alloy and 1Cr21Ni5Ti stainless steel were investigated. The copper-steel composite plate was properly prepared by electron beam welding. The microstructure and composition of welding seam were characterized and analyzed by metallographic microscope. Mechanical properties of welded joints were tested using electronic tensile testing machine and fatigue testing machines. Scanning electron microscopy was used to observe the fatigue fracture in different cycles. The results show that the overall metallurgical bonding of the two metals is good, however, the local fusion zone on the steel side has a large area and extends into the weld from the steel base. The tensile specimens of the copper-steel electron beam welding all fracture at the minimum section of the weld, and the average fatigue limit of the fatigue specimens is 48.04 MPa and they all fracture at the minimum section of the weld. In the high cycle fatigue sample, a single crack source is observed at the endpoint of the weld top surface, while in the low cycle fatigue sample, more crack sources are observed on the top and bottom surface and inside of the weld. The final fracture zone of both samples is located in the copper alloy matrix.It can be seen in the process of fatigue fracture, the number of crack sources is different between high and low cycle fracture, but in both cases, the cracks are easy to initiate at the minimum section of the weld and spread to the copper alloy matrix.