A method to determine the effective shear modulus under shock loading conditions by using longitudinal and bulk sound velocities along the quasi-elastic release was introduced in present paper. The effective shear modulus was calculated for LY12 aluminum over shock pressures range of 20—70 GPa. Results show that the effective shear modulus decreases linearly with the stress, reaching to zero at the reverse yield point, and the effective shear modulus at first release can be depicted by the modified Steinberg-Cochran-Guinan model. Using the effective shear modulus data, simulation has been successful in reproducing the experimental data with quasi-elastic release behavior, showing smooth transition from elastic wave to fully plastic wave.