Through the analysis of the vibration signal, a fault severity assessment method was proposed for the bearings with rolling element damage. Firstly, several parameters can reflect the severity were determined, and their describing ability were examined by experiments. On this basis, based on the Root-Mean-Square (RMS) value, a fault severity assessment method was proposed to refine the damage assessment of rolling element. By extracting the vibration characteristics of different fault severity specimen, the cubic spline curve can be given as the faulty severity assessment criteria. The validity and accuracy of the method was verified by experiment. Data show that, when divided fault severity into normal, minor damage, and serious damage three intervals, the method is able to assess the NJ312 rolling bearing faulty severity.