With the wide application of the electrified railway, high frequency resonances in train-network coupling system often occur, which not only affect the normal operation of trains but also seriously restrict the rapid development of electrified railway. However, the resonance mechanism has not been fully grasped and needs to be solved. First of all, this paper analyzed the new harmonic characteristics of mixed running of alternating current (AC) and direct current (DC) trains in electrified railway. Then, the mechanism of high frequency resonance and the burnout failure reason of the resistor connected with capacitance (RC) branch in the dc train were analyzed. Simulations were carried out to find some basic conclusions and rules. Subsequently, this paper reproduced and analyzed a real case of high frequency resonance accidents of train-network, burnout accident of RC branch of the dc train. Based on the above researches, optimized control methods to avoid the high frequency resonance were introduced. The achievement of this paper provides an important reference and theoretical basis for the comprehensive optimization and control of train-network coupling systems and the resonance suppression measures.