We introduce a phenomelogical model to describe the characters of ferroelectric superlattices.Three kinds of coupling ways are considered in the model.The polarization,Curie temperature and the critical size of the superlattices are obtained by computational simulation.We think that the coupling coefficients are functions related with the temperature and the lattice size.An experience formula is introduced to explain recent experimental results about the KTaO3/KNbO3 superlattice material.