Abstract: With the development of CFD and computer technology, many scholars have begun to use numerical simulation methods to conduct a lot of detailed research on the hydraulic characteristics of stepped spillways, but there is no consistent method for evaluating the accuracy of their calculations.In this paper, three grid sizes ranging from dense to coarse are used to numerically simulate the air-water twophase flow on a stepped spillway by using Flow 3D^? software, to provide a general method to evaluate its calculation accuracy.Firstly, the grid size is optimized by calculating the grid convergence index（GCI）, which can shorten the calculation time while ensuring the calculation accuracy. Then, by using the physical model test measurement results as a reference, the accuracy of the numerical simulation calculation results under the recommended mesh size is evaluated. The results show that using the same mesh refinement factor can greatly simplify the calculation process of GCI. It is recommended using a grid size with a global GCI less than 5%. Although slightly lower than the calculation accuracy for pure water flow, Flow 3D^? software still has high calculation accuracy for the key hydraulic parameters of the air-water two-phase flow on the stepped spillway.