The promising solid-state refrigerator based on the electrocaloric effect plays an important role in the cooling of microelectronic devices.In this paper,a fundamental thermodynamic differential equation considering the polar energy was proposed,and the thermodynamic expressions of heat capacity,temperature,entropy and heat flux in electrostatic field were derived.Then,a verification of the expressions was conducted by using the experimental data of barium titanate,and a good accordance was found.It is concluded that the electrocaloric effect owes itself to the variation of electric dipole moment with temperature and becomes obvious with the increase of temperature sensi- tivity of permittivity.Thus,the research on giant electrocaloric effect in the future should focus on the drastic change region of permittivity with temperature.