The proton exchange membrane fuel cell (PEMFC) stack is made of multiple single cells. Its working life is subject to the performance of the worst cell, which is an obvious short-board effect. Therefore, improving the voltage uniformity is the key factor to improve the working life of the stack. This research determines that during dynamic loading and reduction with sufficient reaction gas, the PEMFC stack will experience a voltage difference of single cell for about 100 ms, which is even more than 100 mV. This phenomenon cannot be observed by the total voltage variation curves of the stack or voltage monitor curve of the single cell with the scan period of seconds. Through theoretical analysis, this paper reveals that the great distribution uncertainty of liquid water in the diffusion layer causes the difference of diffusion coefficients of each single cell. Subsequently, in the fast dynamic response process, oxygen concentrations between the surfaces of the catalysts differ greatly, resulting in the non-uniformity of transient voltage. The voltage difference will disappear again after finishing the dynamic change process, and a new steady process is established. Further experiments indicate that monomer voltage uniformity changes with the current step amplitude and current variation frequency in the dynamic process of variable load. By adjusting the two parameters, we can effectively ensure that the single cell voltage is in uniform change state. This paper reports that substantial non-uniformity exists within a short time during the variable load process of the PEMFC stack. The reason for this phenomenon is also interpreted. A controlling method is also carried out for inhibiting the non-uniformity, which will open a new perspective for controlling the fuel cell stack during the dynamic process of variable load.. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.