The electrolyte of a lithium ion battery is unstable and is easily decomposed at high temperature, which can lead to the degradation of battery performance. To solve this problem, herein a novel electrolyte additive 1-(2-cyanoethyl)pyrrole (CP) has been proposed to improve the electrochemical performance of LiFePO4 batteries at high temperature. The capacity retention of the battery with 1 wt% CP is 76.7%, while that of the battery without the additive is 38.1% after 200 cycles at 60 °C. Theoretical calculation results reveal that the binding energy of CP and PF5/HF is much higher than that of the solvents in the electrolyte. Surface analysis of the electrodes demonstrates that CP can reduce the decomposition of the electrolyte, and restrain the dissolution of transition metals in the electrolyte at high temperature. TEM/XPS results indicate that CP can modify the protective film on the surface of the cathode material and promote the formation of more regular and thinner CEI films. The promotion of the CP additive is of great significance for improving the high temperature performance of lithium ion batteries and is expected to be applied on a large scale.