A numerical MHD code is used to simulate the dynamical responses of Mercury's magnetosphere to different orientations of the interplanetary magnetic field (IMF) and values of the solar wind dynamical pressure. It is found that the IMF orientation is very effective in changing the size of the polar cap and, hence, the surface area subject to direct solar wind sputtering. Even though the polar cap size is not significantly changed by the solar wind dynamic pressure variation for a fixed IMF direction, the global configuration and size of the Hermean magnetosphere are highly responsive. The observed time variations of the atomic sodium emission at Mercury, hence, might indeed be correlated with the solar wind condition.