The ground state energy of the system,which is the electron interaction with both the weak coupling bulk longitudinal optical phonons and the strong coupling surface optical phonons in a polar crystal slab within magnetic fields,are studied by means of the linear combination operator and variational method.The law of the change of the self trapping energy of the magnetopolaron changing with the thickness of the slab and magnetic fields are derived.The contributions of the different branches phonons electron interation on the self trapping energy and the change of it changing with magnetic fields are calculated.