Low frequency resonance mechanism for locomotives and grid of the electrified railway needed to be solved urgently.Based on the impedance ratio on the stability of criterion, influence of all kinds of locomotives and grid parameters for the low frequencyresonance were analyzed, and the important rules and conclusions were found. Finally, reproduced and analyzed the actual low frequencyresonance accident, all the work was verified.An important reference and theoretical basis were provided for fully grasp the low frequencyresonance mechanism and developing appropriate suppression measures.