To investigate the impact of adding parapet walls at the top of the weir and the height of the parapet walls on the discharge capacity of the piano key weir, physical model tests were conducted to compare and analyze the discharge capacity of the piano key weirs with 8 different heights of parapet walls and numerical simulations were conducted to analyze the discharge of each overflow crest, flow pattern and flow velocity of the piano key weirs with 5 different heights of parapet walls. The results show that, compared with the foundation shape, the addition of parapet walls can improve the discharge efficiency and discharge ratio of the inlet and outlet, increase the discharge capacity of the side weir, reduce the overflow collision of the side weir, increase the discharge flow velocity, and as a result, improve the discharge capacity of the piano key weir. When the height of parapet wall is 13% of the weir height, the discharge capacity increases most. When the relative hydraulic head H/P <0.2, the discharge capacity can increase by 8% to 15%. When 0.2< H/P <0.4, the discharge capacity can increase by 10% to 12%.