Urban railway tunnels are an important infrastructure for public transportation. A lining of concrete is often cast to support the ground and protect the internal services in the tunnel. The durability of the concrete lining can be affected by the properties of the local groundwater. The water can flow through cracks or joints in the concrete and then start to attack the concrete from the defect surfaces. The concrete degradation caused by the calcium leaching has an impact on the estimated service life of the tunnel. The propagation of the calcium leached zones could increase the risk of local failure of the lining. A new analytical model for the calcium leaching depth prediction of the lining concrete is presented in this paper. The result shows that the concrete could have a degradation zone for 20 cm around a flowing defect after 20 years of water seeping. An empirical equation based on power regression is also provided for better application.