Abstract: Ship steel structures are always fabricated by welding, while mechanical performance and fracture strength of welded joints significantly influence the strength and service life of the whole ship considering the impact of actual working condition and external loading. In this study, base materials of ship steel plate (Q345 and Q690) were examined by uniaxial tension test to establish strain-stress curves, which can be employed for the assessment of fracture performance. Aiming at Gurson-Tvergaard-Needleman (GTN) failure mode, computational code was programmed and a series of numerical analyses were carried out to investigate the fracture behavior of ship steel plate, while constitutive relations of the examined ship steel plates were proposed with optimized parameters of GTN failure mode. In addition, computation results of strain-stress curves have a good agreement with experimental data. Concentrating on the butt welded joints of ship steel plate with high welding quality, uniaxial tension test was also conducted to obtain stress-strain curves of welded joints. In order to consider the influence of micro welding defects and welding residual stress on fracture performance of welded joints, modified initial void volume fraction and plastic hardening parameters were proposed, and good agreement between computation results and measurements was observed for fracture performance of welded joints.