The microchannel reactor is a generally adopted laminated-sheet structure in which several microchannel sheets with the same or different structures are laminated together and tightened by welding or other connection methods into a single reaction unit. Uniform velocity distribution among the microchannels can improve the heat and mass transfer efficiency. When one or more microchannels are blocked, the uniformity of the velocity distribution is inevitably affected. In this work, the velocity distributions among the microchannels in a single-sheet and a double-sheet laminated structure are investigated under different blockage conditions. It is demonstrated in detail that the different blockage conditions in the microchannels and laminated structure display different effects on the velocity value, the shape of the velocity distribution, and the evaluation parameter for uniformity.