The effective mass of the magnetopolaron in a slab of polar crystal within constant magnetic fields, which is a weak coupling of electron-bulk longitudinal optical (LO) phonons and strong coupling of electron-surface optical (SO) phonons, was discussed by using Tokuda's improved linear combination operator and the Lagrange multiplier and the variational method. The effective mass of the magnetopolaron was obtained as a function of the slab thickness and the magnetic fields. Numerical calculations for KC1 polar crystal slab show that the effective mass of the magnetopolaron first decreases and then increases, finally tends to a stable value with increasing slab thickness, and increases with increasing magnetic fields.