The dynamic deformation behavior and mechanical constitutive of LZ91 magnesium alloy were investigated under high speed impacting using optical microscopy and X-Ray diffraction combing with Schmid factor. The results show that the flow stresses are not sensitive to strain rate and the Portevin-LeChatelier effect occurs. The fine α-Mg phase uniformly distributes in β-Li phase, causing that the dislocation motions are hindered. This finally induces that the adiabatic shear sensitivity decreases and the adiabatic shear band only occurs above 3600s-1. The special texture, which deviates the basal texture nearly 20-30°, in α-Mg phase forms, due to the Burgers orientation relationship between α/β phase interface. Twinning is not required to accommodate the uniform plastic deformation in α-Mg phase because of this orientation relationship. The Johnson-Cook (J-C) mechanical constitutive was modified and the fitting results well match with the experimental results.