Lithium iron phosphate battery (LIPB) is a promising power source for electric vehicle. However, LIPBs generally show a low uniformity in performances due to their poor stability. Herein, we employ an electrochemical method to study the stability of LIPB, and the results show that stable LIPB can be obtained by using the activation with both charge–discharge cycle and aging, since the stable solid-electrolyte-interface (SEI) films are available. The stable SEI films are associated with the low self-discharge of LIPB, characterized by keeping high open circuit voltage (OCV) during storage under discharged states, high capacity, stable internal resistance, and high charge/discharge reversibility. Most of the low-OCV LIPBs can be repaired by using the electrochemical treatment containing charge–discharge cycle and aging. The activation processes with charge–discharge cycle and aging induce the generation of the smooth, adherent, and stable SEI films, contributing to the good stability of the battery. The achievement of the stable SEI films is mainly resulted from the formation of the surface layers completely covering the electrodes' surfaces during charge–discharge cycle, and the stabilization of the layers by transforming the layers' species to the “more inorganic” compounds such as carboxylates or even carbonates (like Li2CO3) during aging.