With both longitudinal optical (LO) and surface optical (SO) modes included, the polaronic effects on the quantum-confined Stark effect in quantum wells is investigated by means of the variation technique. For an infinite and a finite well the significant corrections attributed to LO and SO modes are calculated. It is found that the Stark shift will be weakened by the LO-phonon effect and enhanced by the SO-phonon effect. It is also shown that the influence of electron-phonon interactions depends not only on the well width but also on the potential barrier.