In order to estimate the state of health (SOH) of single cell in battery pack, this paper researched on the battery’s polarization resistance and the Ohmic resistance emphatically. An evaluation methodology of the single cell’s relative SOH was proposed on the basis of battery’s internal resistance, and the SOH of single cell was evaluated using the terminal voltage combining with the internal resistance’s effect on terminal voltage under working condition. and the result analysis of comparative experiment verified that the proposed method had high accuracy and effectiveness.