The thermodynamic properties of ferroelectric thin films are investigated within the framework of the transverse Ising model. A two-dimensional in-plane stress is introduced into the Hamiltonian of the system, and is supposed exponentially decreasing from the interface between the substrate and the film to the surface of the film. It is demonstrated that the compressive stress is benefitial to the polarization and shifts the Curie temperature to higher temperatures, but the tensile stress has the inverse influence on the Curie temperature and polarization. Besides, it is also shown that the diffusive length greatly affects the thermodynamic properties of the film.