Shield tunneling of Changsha Metro Line 5 crossed Liuyang River where the tunnel was covered with shallow overburden and the engineering properties of the whole weathered conglomerate and the upper gravel layer wer poor. Karst is developed in weathered calcareous conglomerate in the tunnel floor with the maximum height of the cavern up to 15.8 m which is filled by loose conglomerate and sandy soil. Before shield tunneling, the fast-controllable sleeve valve pipe grouting technology was used to treat the karst efficiently on a platform built on the river. During crossing operations, several key technologies had been adopted to deal with the complex conditions, such as shallow under water overburden, water rich formation, karst developed floors, closeness to the bridge pile foundation, to avoid risks effectively, and the engineering experience is worthy of reference.