Abstract: Based on the Ginzburg-Landau-Devonshire phenomenological theory, the thickness-dependent ferroelectric and dielectric properties with and without depolarization field in epitaxial Ba_0.5Sr_0.5TiO₃ films were qualitatively studied. The results show that, compared with non-depolarization field, depolarization field makes the spatial distribution of polarization more uniform and inhibits the average polarization of the system, with the phase transition temperature (ΔT_c) reduced but the critical thickness (Δh_c, Δh_m), and tunability (Δφ) increased, especially in the case of weak electrode compensation. With the 100-nm-thickness thin film, the values of ΔT_c、Δh_c、Δh_m and Δφ are about -57.5 ℃, 15 nm, 40 nm and 20%, respectively. Moreover, the dielectric properties of epitaxial films depend on the ability of polarization to respond to external applied electric field rather than polarization itself, which can be fully proved by the completely opposite trends of thickness-dependent polarization and tunability.