Abstract: To investigate the differences in strength and toughness characteristics of single-wire and double-wire narrow-gap submerged arc welded joints of 2.25Cr-1Mo-0.25V steel thick plates used in hydrogenation reactors, mechanical properties were characterized using tensile and low-temperature impact tests. Microstructural analysis was conducted using optical microscopy, scanning electron microscopy, and electron backscatter diffraction to reveal the evolution mechanism of strength and toughness. The results indicate that the impact toughness of the double-wire narrow-gap submerged arc welded joint decreased by more than 40% compared to the single-wire submerged arc welded joint, while the tensile strength and yield strength decreased by 15.7% and 2.6%, respectively. Due to more concentrated heat input, the double-wire narrow-gap submerged arc weld produced coarser bainitic columnar crystal structures. Additionally, compared to the single-wire welded joint, a small amount of austenite, blocky ferrite, and M-A constituents appeared, reducing the deformation compatibility with the bainitic matrix. Furthermore, the content of deformed grains and local misorientation were relatively higher, leading to more pronounced plastic deformation and dislocation accumulation. This behavior tends to accumulate more micro-defects within the material, resulting in reduced mechanical properties such as impact toughness and strength.