Abstract: Structural steel is a widely used engineering material. The evolution of damage from initial defects can significantly affect the safety performance of structures. Taking Q235 steel as an example, the experimental research was conducted on the mechanical properties of materials under uniaxial tension and cyclic loading regimes. ABAQUS was used to establish a two-dimensional microstructure model of Q235 steel considering void defects. Using void expansion model as the failure criterion, simulated analysis was conducted on the mechanical performance of the material for seven different void volume fractions and three different void diameters of microscale initial defects. The results indicate that, the porous single-cell model with random distribution established using the micromechanical finite element method can closely match the experimental results. Under uniaxial tension, the position of the fracture point on the stress-strain curve of the specimen advances with the increase of void volume fraction, the influence of void diameter on the position of the fracture point of the specimen is not significant. Under cyclic loading, the initial void volume fraction has a minor impact on the cyclic hardening phenomenon of the material, the effect of the void diameter on the stress-strain response curve of the material is not significant. Through micromechanical analysis, the impact of initial defects on the mechanical properties of Q235 steel was examined, which can provide micromechanical analysis tools for predicting the mechanical performance of structural steel.