The age structure of aquifer is an important basis for understanding the sustainability of groundwater exploitation. Increasing groundwater pumping with deteriorate groundwater environment in the Muling-Xingkai Plain (MXP) has restricted to some extent the sustainable development of the region. 31 groups of CFCs and 60 groups of ³H samples were collected to delineate groundwater flow systems. Estimated groundwater age ranges from 21 years to more than 70 years in the northern part of the MXP. Younger water occurs in northwest piedmont and hilly areas as well as Muling River, while older water occurs in the central part of the plain and the Wusuli River. This reflects long flow paths associated with regional flow. Although groundwater is mainly recharged by rain and surface water, ground water younger than 10 years was not found for the unsaturated zone consisting of silty clay. The variations of ³H and CFCs combined NO₃^- with wells depth indicate that the aquifer sensitivity of groundwater system above 60m is low and it is high below 100m. Wells for drinking water should exceed 100m.