The relationship between the ground state energy and self-trapping energy (E^{tr}_{e-ph}) with the polar slab thickness of the bound magnetopolaron in a polar slab was studied by using the Huybrecht’s linear combination operator and unitary transformations method. [EQUATION] consists of two parts, one is [EQUATION], the other is [EQUATION], and [EQUATION] also consists of two parts, one is [EQUATION], the other is [EQUATION]. Taking KCl as an example, [EQUATION] and [EQUATION] all reduced with the increase of the slab thickness, and the self-trapping energy was stablized to a low level when the slab thickness was above 5 nm, while when there was a stable magnetic fild, it increased. The reason is probably the change of electron-phonon interaction in presence of stable magnetic field.