The effects of lattice vibration on the properties of the polaron, which was weakly coupled with bulk longitudinal optical phonons and strongly coupled interface optical phonons, in an infinite quantum well were studied by means of the Huybrechts’ linear-combination operator and a modified LLP variational method . The expressions for the self-trapping energy of the polaron in a quantum well as a function of the well’s width and temperature were derived. Numerical results of the polaron self-trapping energy for KI/AgCl/KI QW show that the self-trapping energy of the polaron will decrease with increasing the well’s width and temperature, but the contribution of interaction between the electron and the different branch of phonons to the self-trapping energy and their changing with the well’s width and temperature are different.