In order to reducing spillways construction costs, using Spillway with Converging Walls is cost effective and appropriate solution. Although the use of these type of spillways growing, most studies do not consider effects of the converging walls on flow pattern and structural performance of the spillway. In this study, Flow-3D software and RNG Turbulence model used for numerical simulation of the flow. CATIA software is used to define the geometry of the solid boundaries. A computational block consists of 4000000 calculations used to separating the flow field. In order to verify the numerical model, at first the results of numerical model compared with results of Gavshan dam physical model and after ensuring the accuracy of the numerical model, the effect of Gavshan dam spillway constriction on hydraulic characteristics of the flow modeled with angles of shrinkage 0, 1, 2.5, and 3 in addition to the two-degree angle, which is used in the design of the Gavshan dam, and evaluated for three flow rates of 600, 950, and 1350 cubic meters per second.