The Casimir effect between metal plate and dielectric plate is discussed with 1 + 1-dimensional potential model without using cut-off method. Calculation shows that the Casimir force between metal plate and dielectric plate is determined not only by the potentialV0, the dielectric thickness and the distancea between the metal plate and dielectric plate, but also by the dimension of the vessel. Whena is far less than the dimension of the vessel, the Casimir forceFc∝a-1; converselyFc∝a-2 This result is significant for Casimir force experiment.