On the basis of Huybrechts’ strong-coupling polaron model, the Tokuda modified linear-combination operator method and the unitary transformation method were used to study the properties of the strong-coupling polaron in a polar slab considering the influence of the Rashba effect, which is caused by the spin–orbit interaction, with the triangular quantum well. Numerical calculation on the KCl, as an example, is performed. The expression for the effective mass(m±*) of the polaron as a function of the slab thickness (d), the velocity (u), and the coupling constant (α) was derived. Numerical results show that the interactions between the orbit and the spin with different directions have different effects on the m±* of the polaron.