In situ 5wt.%Al3Zr/6063Al composites were fabricated by direct melt reaction method (DMR) and then processed by deformation pretreatment. Process of the pretreatment is forging of 70% deformation at 450℃ and friction stir processing (FSP). The high strain rate superplasticity of the composites was studied by modern analytic determination methods. The results show that the average grain size of the composites is less than 10 μm after forging and FSP. The composites exhibited superplasticity at the temperature of 350℃ to 500℃ and initial strain rate of 1.0×10-3s-1 to 1.0×10-1s-1. The elongation reached 330% and the sensitive index (m value) is 0.45 at the initial strain rate of 1.0×10-2s-1 and the temperature of 500 ℃. The dominant mechanisms of superplastic deformation were grain boundary sliding and dislocation slip which synergized with moderate grain growth and dynamic continuous recrystallization.