Abstract: In groundwater assessments in northern China, more attention is paid to resources, while less importance is attached to the ecological function and geological environment function of groundwater. In regard to this problem and based on the groundwater circulation system on a catchment scale and natural attributes of groundwater, this paper focuses on coordinate and give play to the resource function, ecological function and geological environment function of groundwater and discusses the internal relation between the groundwater sustainable yield and groundwater functions with respect to their concepts, assessment principle and assessment mechanism. It is thought that they originate from the same concept of harmony between man and nature, take the catchment-scale groundwater circulation system as the research object and the protection of the ecological and geological environment as the target and promote and support each other. The groundwater function assessment is to determine rationally the conditions for the groundwater sustainable yield and the rational determination of the groundwater sustainable yield is the prerequisite for achieving the target of groundwater function assessments. If one of them is lacking, then it is difficult to protect ecological functions or geo-environmental functions of groundwater with a clear aim.