Within the framework of the fraction-dimensional space approach, the binding energy and the effective mass of a polaron confined in a GaAs film deposited on Al0.3Ga0.7As substrate have been investigated. It is shown that the polaron binding energy and mass shift decrease monotonously with increasing film thickness. For the film thickness of Lw70 and the substrate thickness of Lb200 , the substrate thickness will influence the polaron binding energy and mass shift. The polaron binding energy and mass shift increase with increasing substrate thickness. In the region Lw70 or Lb200 , the substrate thickness has no influence on the polaron binding energy and mass shift.