Abstract: Effect of the pier built in the diversion channel on drainage of Dongwei Pumping Station was studied from physical mode test of normal scale. The results show that (a) For frequent floods, a remarkable change of water level nearby the pumping station (rises in the upstream and declines in the forebay) and a deterioration of the flow pattern in the forebay caused by the pier were found. (b) For big floods, the newly-built pier objectively had a regulatory action on the flow, which slightly improved the poor flow pattern in the forebay caused by the design flaws of its own. But together, because the condition under which the pumping station works more frequently is frequent floods, the bad flow pattern caused by the pier impacts the normal and high efficiency operation of the pumps, which brings some potential security risks to drainage of the pumping station.