There is no complete standard method for estimating the exact volume and locations of groundwater intrusion that may occur in rock tunnels. The Tunnel Site Water Intrusion Rating (SGR) system is based on preliminary field surveys to quantitatively classify tunnels from the perspective of groundwater intrusion. In this rating system, parameters such as jointing, schistosity, crushed areas, karstification, soil permeability, tunnel water head, and annual rainfall are evaluated. However, this method needs to be improved according to geological and hydrogeological conditions. Some parameters should be modified or even eliminated, and some new parameters should be proposed. In this paper, the SGR method is optimized according to the new parameters. In this research, the accuracy of the SGR method was investigated based on the actual excavation experiences of Kerman and Karaj tunnels. Two parameters were eliminated and other parameters were changed. As a result, the optimized SGRM method is introduced. The SGRM results are closer to the actual data obtained from the above tunnels compared to the original SGR results. Therefore, this method is introduced as a new method for estimating water inflow into the tunnel.