Abstract: Based on the uniaxial tensile testing of small size specimen made from domestic A508-III steel, a macroscopic mechanical constitutive model and a ductile damage model were established, and the crystal plasticity parameters at the microscale were calibrated. A multiscale numerical model for the uniaxial tensile behavior of small size specimen was established by combining macroscopic finite element method with microscopic crystal plasticity. The tensile mechanical response behavior of small size specimen was explored at the macroscopic scale, and the plastic deformation mechanism was explained at the microscopic scale. The results show that the tensile testing of small size specimen exhibits a certain discreteness and the tensile fracture surface shows obvious ductile fracture characteristics. At the initial stage of tension, plastic deformation is primarily achieved through the uniform dislocation motion. As strain increases, the strain localization and stress concentration at the grain scale become evident at the microscale, especially after necking, the non-uniform plastic deformation becomes more significant, and dislocations begin to show obvious localization phenomena. The geometrically necessary dislocation (GND) density increases rapidly from 16 μm⁻² at a strain of 8% to 65 μm⁻² at a strain of 10%. Throughout the plastic deformation process, the evolution ofstatistically stored dislocation (SSD) density plays a dominant role. Due to the uneven stress distribution and dislocation pile-up, an "orange peel effect" is observed on the specimen surface. Grain boundary have a significantly impact on the dislocation evolution, and the dislocation accumulation at grain boundary follows the rule that the larger the grain misorientation, the higher the dislocation density.