Abstract: Piano-key weir (PKW) is an innovated labyrinth weir that was proposed at the beginning of this century. It has a greatly-reduced base owing to its overhanging shape, and can be placed on all types of dams. With a super discharge capacity, it is an effective solution to the dams that have an insufficient flood-discharge ability. The excellent discharge capacity of PKWs has been verified by previous laboratory experiments, but the results did not distinguish the contributions of different overflow edges to the total capacity and the corresponding complex flow features are not fully understood yet. This paper presents an analysis on the overflow characteristics of PKWs using numerical simulations of 3D turbulent flows and VOF free-surface capturing. Results show that the simulated discharges agree well with experiments. At low head the discharge capacity increases almost linearly with the length of overflow edge, while at high head it decreases gradually. We obtained the discharges over the outlet, inlet and side edges of the PKW, along with their proportions in the total discharge, analyzed the relationship of discharge efficiency versus head rising, and examined the mechanism of variations in discharge efficiency with flow patterns. This work would deepen our understanding of discharge characteristics of piano-key weirs at different overflow edges and suggest possible directions of optimizing piano-key weir shape.