Abstract: Downstream submergence can determine the relationship between the water head and the discharge capacity of the weir, once the tail water exceeds the weir crest elevation in the channel application of weirs. This study aims to explore the discharge characteristics of Piano Key Weirs (PKW) under submerged flow conditions. A series of physical tests were carried out for the four types of PKW in a rectangular flume. The relationship between the total head on the upstream/downstream and the submergence coefficient of PKW flow was obtained in the variation process with the downstream water level under the submerged flow. The flow reduction coefficient and dimensionless water head method were also used to determine the relationship between the total head and flow reduction coefficient of PKW and the submergence coefficient. An analysis was made on the sensitivity of four PKW types to submergence. The fitting was performed on the formula of submergence flow reduction coefficient of PKW. The reverse standardized upstream water head method was utilized to compare the hydraulic efficiency of four PKWs under submerged conditions. The experimental results show that the impinging jet, breaking (surface jumping), surface wave, and surface jet flow after the four kinds of PKWs appeared in sequence, as the downstream water level rose. Specifically, the type C and D PKWs were gradually submerged at the submergence coefficient S=0.6, when the upstream discharge was constant. The discharge reduction coefficient decreased with the increase of the submergence coefficient. The type B and A PKWs entered the submergence state in advance, when S≥0.15 and S≥0.2, respectively. Correspondingly, the critical submergence Sm of four types of PKWs were: type A 0.5, type B 0.3, type C 0.7, and type D 0.65, respectively. In contrast, the type B PKW with only upstream overhang structure was the most sensitive to the submergence, followed by type A with the symmetrical upstream and downstream overhang structure, while the overhang structure towards the downstream was delayed the upstream submergence. Different PKWs varied differently under the downstream water level, which was mainly related to the overhang length of the upstream and downstream of each model. The longer the overhang in the upstream was, the slower the slope of the outlet key was, and the more likely it was to be submerged. In the type C and D PKWs with the longest downstream overhangs, the water nappes flowing through the inlet key under the action of the inertia of the water flow, pushed the downstream water flow to a position far away from the PKW crest, thus delaying the effect of submergence. Finally, the calculation formula of submerged discharge coefficient was fit to further compare the discharge capacity and hydraulic efficiency of four types of PKWs under submerged conditions. It was found that the discharge efficiency of type C and D PKW was lower than that of type A PKW, and the efficiency of type B was the highest when the submerged coefficient S was relatively small. There was a reverse variation in the discharge efficiency of each type of PKW, when S≥0.7. The submerged discharge efficiency of four types of PKW depended on the comprehensive influence of "sensitivity to submergence" and free-flow discharge efficiency. The findings can provide technical support for the optimal weir flow of the PKW in the application of open channels and rivers.