Abstract: The biogeochemical process in the subterranean estuary of a coastal zone is an important driving force to the interaction between land and sea, which is critical for the research of material budget. In this study, the Qingdao Golden Beach was chosen as a typical case for research. Using resistivity imaging and isotopic tracing technology as means, we tried to understand the biogeochemical changes of the subterranean estuary on a tidal scale. It is found that in a typical tidal cycle, various physical and chemical parameters of groundwater, such as resistivity, salinity, pH, radium/radon isotopes and nutrients, are all significantly influenced by the changing tide. Tidal signals in the subterranean estuary are usually delayed for about 2-4 hours. In the cycle of flood and ebb tides, seawater will be recharged into and discharged out of the subterranean estuary, and thus the biogeochemical background of the aquifer will be significantly changed.