With both the electron-longitudinal optical phonon and the electron-surface optical (SO) phonon interaction included, the temperature dependence of the effective mass and self-energy of a polaron in a polar crystal slab is investigated by means of the Lee-Low-Pines variation method. These quantities are calculated for GaAs as an example. It is found that both the effective mass and the self-energy will decrease with increasing temperature. In particular, for a very thin slab, the temperature dependence of the effective mass is nearly entirely determined by the SO phonon effect.