The time-variable structure of Ganymede's magnetosphere is studied by means of resistive MHD simulations. Using the magnetometer measurements of the Galileo spacecraft in the first few Ganymede flybys as examples, we find that the plasma flow pattern inside the Ganymedian magnetosphere could be subject to significant changes. Furthermore, the boundary of the polar cap dividing the open magnetic field region from the closed magnetic field region could expand or contract as the Jovian magnetic field swings inward and outward. Such an effect could be related to the Hubble Space Telescope observations of large time variability of the atomic oxygen airglow near the polar regions of Ganymede.