Using three-dimentional model，the hybrid particle in cell method is used to simulate the inflation of magnetic field by injecting plasma into dipole magnetic field. Two different types of magnetic field expansions are considered：the high density plasma is injected into a dipole magnetic field with or without a background magnetic field. It is found that the existence of background magnetic field not only changes the particle distribution，but also significantly changes the extent of magnetic field inflation. The effects of plasma injection velocity on the magnetic field inflation are also studied. It is shown that the higher the injection velocity， the larger is the size of the inflated magnetic bubble. For the lower injection velocity，the gyroradius of the injected particle is small compared with the dipole field scale length， so that the particles are trapped in the dipole field and the effect on dipole magnetic field can be ignored.