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The stability region of multi-voltage level DC system is the bottleneck problem in system level stability analysis. Based on the Brayton Moser mixed potential function theory, the mixed potential function of a simple three-stage cascade system is constructed, and the unified simplified models of the system under different control modes is obtained. Then, according to the stability theorem of mixed potential function, the large signal stability criteria of the system under different control modes are proposed. Through further analysis, the key parameters affecting the large signal stability of the system are obtained to draw the large signal stability region of. Finally, the simulation results verify the feasibility of the proposed large signal stability criterion.