The fault model effect and criticality for the traction power supply system of the electric multiple unit (EMU) is studied to improve the reliability of both the traction power supply system and the high speed train operation. The fault model effect and criticality analysis (FMECA) method is adopted, the reliability block diagram of the EMU traction power supply system is put forward, and 16 kinds of failures of the traction power supply system are studied. The traction power supply system hazard matrix is obtained from the two dimensions of failure probability and impact severity. Through analysis of this matrix, four important failures are identified that have the highest hazard degree: malfunction of the IGBT monitor module, trip off of transformer oil pump 34-F55, failure of the refrigeration fan, and failure of transformer oil flow. Meanwhile, some corresponding correcting actions are adopted. Feedback results show that the above measures ensure the operational reliability of the traction power supply system. The research outcome also provides useful support for other EMU subsystems in failure analysis, maintenance decision-making and life-management as well.