Piano key weirs, as a type of non-linear spillway with specific geometry, have a higher discharge capacity and have thus attracted researchers' attention. The use of piano key weirs is proposed as an alternative solution to improve dam performance. In this research, the performance of piano key weirs was studied using numerical simulations in the Flow 3D software. After evaluating the accuracy of the numerical model results with the experimental results and ensuring the high accuracy of the numerical model, the development and optimization of the numerical model to enhance the spillway's performance in passing flow begins. Based on earlier research, a notable drawback of these spillways is the decrease in the discharge coefficient as the upstream volume flow rate increases, which causes an increase in flow head before the spillway. Therefore, in this research, geometric modifications to the spillway model, including sloping the sidewalls of the spillway by 5, 7, 10 and 12 degrees, were examined. The numerical modeling results indicated an 11% to 15% increase in the discharge capacity of the piano key weir with 5-degree sloped walls, an 8% to 19% increase in the discharge capacity with 7-degree sloped walls, and a 7% to 11% increase in the discharge capacity with 10-degree sloped walls compared to the simple spillway at various upstream volume flow rates. However, in the model constructed with a wall slope of 12 degrees, no increase in the discharge coefficient was observed, and the flow head upstream of the spillway increased.