Lead-free K _1–xNa _xNbO ₃thin films, as a candidate for sensors and electromechanical and electrocaloric cooling devices, have increasingly received attention. However, for (111)-oriented films, the relation between phase transitions and electrocaloric effect is not clear. Here, we derive the thermodynamic potential of (111)-oriented thin film ferroelectrics K _1–xNa _xNbO ₃based on the 8 ^thorder polynomial function, and then establish the temperature-misfit strain and out-of-plane stress-in-plane misfit strain phase diagrams and calculate electrocaloric (EC) entropy changes Δ Sand temperature changes Δ T. This study focuses on mechanical and orientation controls of room-temperature EC effect of K _0.5Na _0.5NbO ₃films, which is critical for environmentally friendly electrocaloric refrigeration applications in practice. Under the stress-free and zero misfit strain conditions, the (111)-oriented K _0.5Na _0.5NbO ₃film in an electric field of 30 MV/m has a maximum EC Δ Tof ～18 K near the rhombohedral ferroelectric-paraelectric phase transition temperature (about 673 K). However, an out-of-plane stress of about –6.7GPa can reduce the optimal operating temperature to room temperature where the K _0.5Na _0.5NbO ₃film has the EC Δ Tof ～7.5 K under the action of applied electric field of 30 MV/m. The present work provides theoretical guidance for exploring the strain engineering and orientation engineering of K _1–xNa _xNbO ₃-based thin films with optimized electrocaloric and electromechanical properties.