To investigate the detwinning behaviors and dynamic mechanical properties of a precompressed rolled AZ31 magnesium alloy sheet impacted under high strain rates, the as-received sheet was precompressed along the rolling direction (RD) to the true strain of 4% for inducing { \mathrm{10}\overline{\mathrm{1}}\mathrm{2}} tensile twins. The as-received and precompressed rolled AZ31 magnesium alloy sheets were impacted along the normal direction (ND) using a split Hopkinson pressure bar experiment apparatus at strain rates of 700, 1000, 1300, and 1600 s^-1. Microstructural characteristics of the as-received, precompressed, and impacted specimens were analyzed and compared by an electron backscatter diffraction technology. The results show that in the precompressed specimen, the density of the basal texture was weakened and a new twin texture with the c-axis paralled to RD was formed. The average grain size of the precompressed specimen decreased visibly as a result of the parent grains being subdivided by tensile twin boundaries. The dominant deformation mechanism of the precompressed rolled AZ31 magnesium alloy impacted along ND is detwinning. With increasing the strain rate, the initial basal texture recovered, the average grain size increased, and the average twin thickness decreased. Compared with the precompressed specimen, the as-received specimen impacted along ND exhibited higher strength and lower formability. The precompressed specimen demonstrated greater strain rate sensitivity during plastic deformation.