Nanometer sized Al₂O₃ reinforced Al-Zn-Mg-Cu matrix composites were subjected to treatments in high pulsed magnetic field with different magnetic induced intensity 2T, 3T and 4T. The results demonstrate that the residual stress arrives to a minimum of -1MPa by an applied 3T pulsed magnetic field, which decreased by 102.4% compared to that of the original composite. The applied magnetic field can relax the long range distance stresses between areas with dense and sparse dislocations respectively; Meanwhile, the magnetic field increases the mobility of dislocations and accelerate the release velocity of internal stress, then the residual stress is, thereafter, lowered. The tensile strength increased with the enhancement of magnetic induced intensity. By 4T magnetic field the introduced mass factor, which is a combined parameter to represent the tensile strength and elongation, was enhanced by 12.7% compared to that of the original composite. The high dislocation density is beneficial to the dislocation induced strengthening. Besides, an other important reason lies in that the applied magnetic field may facilitate the formation of metastable η'(MgZn₂) phase as the main precipitates, which somewhat substitute the common η (MgZn₂) phase. Thereby, the increase of η'(MgZn₂) can improve the strength and toughness of composites. Furthermore,based on the first principle the density of electron spin state is calculated, which corresponds to the bonds formation process. By 2T magnetic field treatment, the fractograph of the composite exhibits the characteristic of ductile fracture that corresponds to a higher elongation of 9.3%, which is 12% higher than that of the original composite.