The invention relates to a method for online evaluating the state of health (SOH) of a lithium ion battery. The method online identifies an important parameter, namely the ohm internal resistance of the lithium ion battery, indicative of the SOH and the operating state of the lithium ion battery by using a recursion least square identification algorithm based on a battery equivalent circuit model, introduces a method for screening the internal resistance at preset time interval in view that the ohm internal resistance of the lithium ion battery is in close connection to battery pack temperature, current power, and battery state of charge, establishes a relation between the screened mean internal resistance and the SOH, and accurately reflects the current SOH of the lithium ion battery by using a battery internal resistance characteristic so as to further improve the use safety of the lithium ion battery.