Abstract: To reveal the mechanical property weaknesses and plastic deformation mechanisms in the X65 pipeline steel welded joint, micro-tensile testing was combined with Electron Backscatter Diffraction (EBSD) technology in the present study. A systematic investigation was conducted on the microstructure, tensile properties, and in-situ plastic deformation behavior of the base metal (BM), weld metal (WM), and heat-affected zone (HAZ) within the welded joint. It was found that significant microstructural differences existed among the various zones of the welded joint due to differing thermal cycle histories during welding. The BM was characterized predominantly by uniformly distributed acicular ferrite. The WM was composed of fine ferrite, while the HAZ exhibited a coarse ferrite-pearlite mixed microstructure resulting from grain coarsening. Tensile property tests revealed that the BM possessed the highest ultimate tensile strength (UTS) (610 MPa) along with excellent plasticity. The HAZ exhibited the lowest UTS (approximately 530 MPa) and the poorest plasticity. The properties of the WM were observed to be close to those of the BM. In-situ EBSD analysis was employed to quantify the evolution of plastic deformation in the WM zone using Grain Reference Orientation Deviation (GROD). Under no load, the GROD values were low and uniformly distributed. With increasing displacement, the GROD values progressively increased and exhibited heterogeneous distribution. Ultimately, extensive areas of zero solutions emerged due to severe lattice distortion causing the disappearance of electron diffraction patterns. Fractographic analysis demonstrated that uniformly sized dimples, indicative of typical ductile fracture, were present on the fracture surfaces of both the BM and WM. In contrast, the fracture morphology of the HAZ displayed dimples of non-uniform size accompanied by cleavage facets, revealing a ductile-brittle mixed fracture mode. This study elucidated the regulatory mechanism of microstructure on the mechanical properties of the welded joint. The findings provide a theoretical basis for optimizing welding processes and ensuring the service safety of pipelines..