The Accelerated Pole Power Supply (AGPS) provides an ultra-high-voltage DC power supply for negative ion neutral beam injection (N-NBI). The high-voltage uncontrolled rectifier in AGPS employs multiple low-voltage diodes connected in series to achieve high blocking voltage capability. However, the physical nodes between the series-connected diodes are accompanied by distributed capacitances to the high-voltage terminal and ground, which influence the dynamic voltage behavior of the diodes. Particularly under step-wave excitation with high dv/dt falling edge reverse voltage, unbalanced potential changes occur at the physical nodes, leading to diode voltage imbalance and false conduction phenomena. The present paper proposes a mathematical model to describe the transient voltage characteristics of the diodes under such excitation, and to quantify the influence of distributed capacitances. The validity of the proposed conjecture and the accuracy of the mathematical model are confirmed by simulation experiments. The findings of the present study demonstrate that the effect of distributed parameters on series-connected diodes cannot be overlooked, as the induced false conduction increases power losses. This study provides significant insights into the application of series-connected diodes.