Abstract This paper investigates the cold start characteristic of the polymer electrolyte membrane fuel cell stack in the constant voltage startup mode. The stack is used in actual vehicle working conditions, so it is practical significance to study the cold start characteristics of the stack. The evolution of the temperature and current density distribution at different regions in the constant voltage mode of the stack are studied. Consistency analyzes from two dimensions, in-plane and through-plane are studied. Then, the performance degradation of the stack after the cold start is explored through the polarization curve and the voltage consistency. The cold start characteristics and performance degradation mechanism of the stack are found. The stack has better cold start performance than that of the single cell, which can realize a rapid cold start at −15 °C in 95 s. Also, the performance of the single cell in the middle position is obviously better than that of the single cell on both sides. The internal behavior of each single cell during cold start process is different and the performance consistency is poor after the failed cold start. The performance degradation gradually decreases from the single cell near the inlet to the single cell far away from the inlet. This paper can provide direction for the optimization cold start strategy.