Dynamic yield strength is one of the key variables in construction of the constitutive relationship for solids under shock loading. This paper reports the yield strength of LY12 aluminum alloy which was measured experimentally on the basis of the method proposed by J. R. Asay and L. C. Chhibildas, in which the test sample underwent a load-reload process and a load-unload process, and the particle velocity history at the LY12 Al sample/LiF transparent window interface was observed by velocity interferometer system for any reflecting surface (VISAR). Improvement in fabrication of the layered flyer plate made these experiments successful. The measured yield strength data at shock stresses of 22GPa and 34GPa are consistent with the reported data at relatively lower pressures. Moreover, physical mechanism of the elastic precursor emerging in the reloading-process was discussed briefly. It is concluded that dislocations is the principal factor resulting in the elastic precursor.