Abstract: Magnetically driven flyer plate experiments, shot 151 with 370 m thick aluminum flyer plate, carried out in the large current pulse device PTS are simulated and anlayzed with the two-dimensional magneto-hydrodynamics code MDSC2. Numerical simulation shows that the material of the whole flyer plate melts or evaporates due to ablation. Density of the material near the free surface and the loading surface of the flyer plate is low, however, that of the remaining part between the free surface and the loading surface is high. This finding tells us that the velocity measured by VISAR in the experiments may not be the velocity of the liquid-solid interface but be the velocity of the solid density reflecting interface, since the laser of the VISAR will penetrate through the free surface part whose density is lower than that of the solid density reflecting interface and reflect back at the solid density reflecting interface. This idea is confirmed by the coincidence of the simulated velocity of the solid density reflecting interface and the velocity measured by the VISAR.