In this paper the reflection index of an unsteady magnetized plasma slab irradiated by an electromagnetic wave is investigated. The direction of the external magnetic field is normal to the plasma surface. Here, it is assumed that the electromagnetic wave is normally incident on the gas slab. On the other hand, this gas slab is being ionized by another strong microwave field and the density of the unsteady plasma increases with time. The effects of the density growth rate, the thickness of the plasma slab and the external magnetic field on the reflection index are simulated.