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The electric resistance welding of thin-thickness duplex stainless steel (DSS) has an outstanding attraction in the plate-fin heat exchanger which is widely used in power machinery, diesel locomotive and other fields, but its structural integrity is threatened seriously by fatigue failure. Therefore, in this research, the formation of the heterogeneous microstructure of DSS electric resistance welding joints (ERWJ) was analyzed by the optical microscope (OM) and electron back-scattered diffraction (EBSD) observations, and the uniaxial tensile and fatigue tests were subsequently executed to disclose their mechanical properties. The intrinsic relevancy between phase-related characteristics and failure behavior was extensively analyzed. The results show that the ferrite was dominating at whole ERWJ, particularly at the energized area, accounting for up to 72%. More than 45% of high-angle boundaries (HABS) and the highest kernel average misorientation (KAM, 2.46° for austenite and 1.52° for ferrite), which indicates greater plastic deformation and residual stress, were observed at the weld edge. In addition, the failure occurred as a tear along the weld edge regardless of the monotonic and cyclic loadings under the deterioration of residual stress and island-like austenite. The distribution of fatigue life with cyclic amplitude performed an obvious turning point of 19.5 MPa due to the different failure mechanisms, providing a permissible range of the stress amplitudes and reinforcement strategies for the plate-shell heat exchanger ERWJ in the engineering. [ABSTRACT FROM AUTHOR]