Abstract: To investigate the plasticity and ductile fracture behavior of Q355 structural steel, a series of experimental and numerical studies were conducted, including a standard uniaxial tensile test and tensile tests of seven groups of notched specimens under different stress states. Finite element simulations were then employed to explore the full-range true stress-strain relationship and the applicable ductile fracture criterion. Firstly, the uniaxial tensile behavior of Q355 was simulated, and the true stress-strain relationship under uniaxial tension was validated by comparing the finite element results with the experimental results. Based on the calibrated true stress-strain relationship, the tensile test of the notched specimens was further simulated. The results show that under shear-dominated stress states, the use of the Mises yield criterion tends to overestimate the material strength. Therefore, a strength reduction coefficient was introduced into the equivalent Mises stress circle to modify the calibrated true stress-strain relationship. The simulation results using the modified stress-strain curves show good agreement with the experimental results. Subsequently, based on the simulation results of the tensile tests, the parameters of the Modified Mohr-Coulomb (MMC) ductile fracture criterion were calibrated to predict the ductile fracture behavior of Q355. The predicted fracture load error was within 6%. This study provides an effective analytical framework for the numerical simulation of the plastic and ductile fracture behavior of Q355 structural steel.