Abstract: A572 Gr.50 thick plates are commonly used in large-scale plate girder fabrication. The welding cracks are likely contained in the butt weld of A572 Gr.50 thick plates. To evaluate the safety of welding defects of the in-service large-scale plate girders in boiler steel structures, A572 Gr.50 thick plate butt-welded joints were fabricated to reflect the similar material properties of the butt-welded lower flange plate of large-scale plate girders. Uniaxial tensile tests, Charpy V-notch tests and three-point bending tests were conducted for the base metal, heat-affected zone and the butt weld of this simulated welding joint respectively. Combined with the finite element analysis, the fracture properties of A572 Gr.50 thick plate butt-welded joints were investigated. Results indicated that materials from base metal, butt weld and heat-affected zone showed good mechanical properties, satisfying the relevant specification requirements, and possessed good capability of plastic deformation. Charpy impact energy reduced with the decrease in temperature. In addition, the base metal, weld and heat affected zone had good impact toughness. With respect to the low-temperature brittleness, the brittle resistant ability of the base metal was the best while that of weld was the worst. Compared with base metal and heat-affected zone steel, fracture toughness of molten zone was still the worse. To summarize, more attention should be paid to the butt weld zone of the thick-plate welding joints for their safety assessment. Finite element software packages can be used to analyze the fracture properties of steel structural members with cracks under working conditions to ensure structural safety.