Lead-free ferroelectric electrocaloric ceramics that could convert electrical energy into heat are the promising candidate for environment-friendly cooling devices. For refrigeration devices, a large temperature change (ΔT) and good temperature stability are required, which are highly related to the phase structure and the applied electric field. In this work, a diffused ferroelectric–paraelectric (FP) phase transition is formed in (K, Na)NbO3 (KNN) by using appropriate composition engineering. The relaxor ferroelectrics in this work present both a large ΔT of 1.24 K and a high ΔT/ΔE of 0.19 K mm/kV. In addition, a wide temperature span exceeds 55 °C at the high electrocaloric effect (ECE) criterion (ΔT ≥ 0.5 K) could also be observed. This work not only opens a new strategy for obtaining high-performance ceramics for refrigeration devices but also extends the application area of the KNN-based lead-free ferroelectrics from sensors, actuators and energy harvesting to solid-state cooling applications.