This work aims to reveal the process of microstructure evolution as well as the atomic-scale mechanism of plastic deformation and residual stress in laser-shocked aluminium alloy, which could provide the theoretical guidance for improving the mechanical property of aluminium alloy by the laser shock. Here, a piston method based on molecular dynamics was used to simulate the laser-shocked polycrystalline aluminium alloy (Al-Mg-Zn-Cu) with different shock velocities. The response of aluminium alloy during the shocking-holding-unloading process was expounded in detail. The microstructure evolution and dislocation distribution, as well as the influence of laser shock on the mechanical properties of aluminium alloy, were analyzed by common neighbor analysis and dislocation extraction algorithm method. The piston velocity was found to greatly influence the laser-shocked aluminium alloy, especially the high piston velocity. With a piston velocity (Up) of 1.0 km/s, sliding motion of the systems become active and the corresponding dislocation density increased. Partial dislocation and stacking faults were converted to perfect dislocation during the holding process, and dislocations mainly distributed around the grain boundary. Meanwhile, the total dislocation density of the system kept stable during the shocking-holding-unloading process, with the Shockley dislocation transformed into other dislocations. High dislocation density was distributed at both ends of the laser-shocked aluminium alloy, leading to the plasticity deformation in the grain and grain boundary. Due to this plastic deformation, the residual compressive stress appeared at the surface of laser-shocked aluminium alloy, and the maximum residual stress was up to 1.2 GPa. A uniaxial tensile simulation of laser-shocked aluminium alloy further showed that the ultimate stress of aluminium alloy was almost not affected by the laser shock with Up=0.3 km/s. However, the ultimate stress was increased by 15% and 22% in laser-shocked aluminium alloy with Up=0.7 km/s and 1.0 km/s, respectively. The residual compress stress of laser-shocked aluminium alloy cancels out part of the external tensile stress at both ends of model, resulting in an increase of the global tensile stress. Meanwhile, the increasing dislocation density and the migration of grain boundary indicated the plastic deformation at the yield stage of laser-shocked aluminium alloy, which led to the improvement of the ultimate stress. In summary, the microstructure and mechanical property of polycrystalline aluminium alloy (Al-Mg-Zn-Cu) is notably influenced by laser shock with moderately high impact velocities. Aluminium alloy has a high dislocation density at both ends of the model after high shock velocity. High dislocation movement induces the plastic deformation of grain and grain boundary, resulting in the residual compressive stress at the surface of aluminium alloy. When a uniaxial tension is applied, the plastic deformation of laser-shocked aluminium alloy at the yield stage is mediated by increasing dislocation and grain boundary movement of the deformed grain boundary, finally resulting in the improvement of mechanical property.