The electrocaloric effect (ECE) of ferroelectric materials, which occurs significantly in a narrow temperature region near the first-order paraelectric/ferroelectric transition (FOPFT) Curie temperature, has great potential in solid-state refrigeration. Most ferroelectric materials, however, bear the second-order paraelectric/ferroelectric transition (SOPFT). In the present study, we demonstrate a size-dependent pseudo-first-order phase transition (PFOPT), associated with ultrahigh ECE and Curie temperature in ferroelectric nanoparticles with degradation layers by employing phase field modeling. The PFOPT behavior of the polarization component P₃ along the applied electric field direction x₃ versus temperature is similar to the classical FOPFT behavior. The results indicate that the ultrahigh ECE and PFOPT occur at temperatures below the Curie temperature of a given nanoparticle size. The adiabatic temperature change is 3.347 K in the simulated barium titanate nanoparticle of 10 × 10 × 8 normalized size under a 96.502 kV cm⁻¹ applied electric field change. The concept of PFOPT should be general, applicable to all ferroelectric perovskite materials. Therefore, the current results provide a novel physical perspective for experiments and for lower power/higher efficiency solid-state cooling devices.