The invention relates to a method for detecting the state of health of a lithium ion battery online. According to the method, based on a simplified energy balance model and a lithium ion battery real-time operating curve, two characteristic parameters, namely, internal resistance and average polarization voltage of a corresponding battery are calculated out conveniently through a series of numerical value fitting methods, and the state of health of the battery which operates online is judged in real time according to the two characteristic parameters and a standard database.