Abstract: Magnetically driven flyer plate experiments, shot 151 with a 370 m thick aluminum flyer plate and shot 164 with a 330 m thick aluminum flyer plate, carried out in the large current pulse device PTS are simulated and analyzed with the two-dimensional magneto-hydrodynamics code MDSC2. Numerical simulation shows that the material near the free surface of the flyer plate melts or evaporates due to ablation, with the remaining part between the free surface and the loading surface staying solid. This finding tells us that the velocity measured by VISAR in the experiments may not be that of the free surface as expectation but be the one of the liquid-solid interface beyond the free surface, since the laser of the VISAR will penetrate through the melt or evaporated part and reflect back at the liquid-solid interface. This idea is confirmed by the coincidence of the simulated velocity of the liquid-solid interface and the measured one by the VISAR.