During the tunnel’s construction, pre-inject to form loop in wall rock of the fault fracture and karst-developed area where there is abundant water. To reduce the quantity of seepage for lining, simultaneity, set up drainage system to drain the rest of water to achieve its purpose that reduce the press of external water and control drainage with effort. According to the principle of elastic-plastic, the paper analyses and designs the thickness optimization based on the status change of plastic zone during excavating tunnel. The result indicates that the pre-injection loop of the single electrified railway tunnel with standard V level is ranging between 4m and 8m when leave out of consideration of external water load. Otherwise the thickness optimization is around 6m for the deep-buried 200m tunnel.