Pressure is, like temperature, a basic thermodynamic variable which can be used to alter the matter state. The atom volume, free energy of matter and other physical and chemical properties can be changed due to the application of high pressure. Many interesting materials including superconducting, super-hard, amorphous, nano-materials can be prepared under high pressures. Meanwhile, the application of high pressure during solidification of metallic materials has also attracted much attention of researchers in recent years. However, the understanding of high pressure on alloy solidification behavior is still lacked, and needs more experimental and theoretical investigation. In the present work, the effect of high pressure on solidification microstructure, phase constitution and mechanical properties of Al-20Mg alloy was investigated by OM, XRD and tensile test. Influence of solute distribution on mechanical properties of solid solution was analyzed and the corresponding mechanism was discussed based on the solute strengthening theory. The results showed that the amount of intermetallic compound b-Al₃Mg₂ decreases and the amount of Al(Mg) solid solution increases in the Al-20Mg alloy solidified under high pressure, resulting in the remarkable enhancement of the mechanical properties. The Al-20Mg alloy is fragile under 1.0×10⁵ Pa. However, it can transform to be a ductile material with elongation of 11% when solidified under 2 and 3 GPa. Meanwhile, its strength can be also greatly improved. The ultimate tensile strength of Al-20Mg alloy solidified under 2 GPa is 8.9 times of that solidified under 1.0×10⁵ Pa. The yield strength of Al-20Mg alloy solidified under 2 GPa is higher than that under 3 GPa. This phenomenon was explained by solute strengthening theory, and proved that the inhomogeneous distribution of Mg solute in the solid solution can enhance the mechanical properties. The fracture characteristic is essentially altered under the condition of high pressure solidification. The Al-20Mg alloy is cleavage fracture under 10⁵ Pa, however, it transforms to the dimple fracture when solidified under 2 and 3 GPa. The present work provides a potential route to improve the mechanical properties of solid solution through the control of solute distribution in the solid solution.