The service life of FeS2 thermal batteries is significantly affected by self-discharge of the cathode. Herein, SEM, XRD and XPS were employed to characterize the mechanism of self-discharge of the FeS2 cathode. A novel combined-discharge method, in which a tiny current (5 mA cm-2) was applied to minimize the effect of polarization on discharge capacity, was conducted to study the kinetics characteristic of self-discharge of FeS2 cathode upon discharge. Then, the self-discharge kinetics parameters which are related to the current density (20, 50 and 200 mA cm-2) and temperature (400, 450, 500 and 550 °C) were determined by the Serin-Ellickson model. Characterizations of the cells standing at 500 °C confirm that the decomposition product of the FeS2 cathode is FeS. The quantitative analysis of self-discharge rate constants (SRC) demonstrates that the reaction is a diffusion-controlling process. The kinetics process can conform to the Serin-Ellickson model. Specifically, the values of SRC increase when the cell is carried by a heavier load, since more breakage would form in FeS2 particles at the larger current density. Besides, the SRC increase at a higher temperature, and the relationship of SRC and temperature can be fitted by the Arrhenius equation. Consequently, the apparent activation energy decreases with the increase of current density.