The growth of the global population had led to a sharp increase in water demand in recent decades. In general, groundwater flow is very slow (1 m/a or less) and its velocity is difficult to measure directly. The isotope method of groundwater dating has been used to estimate the average age of groundwater in wells. As dissolved inorganic carbon (DIC) is almost ubiquitous in groundwater, carbon-14 is a widely used groundwater radiation dating technology. This study conducted the dating analysis of carbon-14 and tritium in seawater using its characteristics of indication age, retrospective source and path. Water samples were collected and sent to laboratory for analysis of C dating and tritium dating, in order to clarify the age of groundwater in Penghu area and the source of recharge, explore the water mixed with old water by near-range vertical recharge, distal lateral replenishment or new water (<64 years), and estimate the rate of groundwater replenishment. The results showed that the shallow groundwater in study area was modern water, suggesting that rainfall infiltration was the main source of groundwater recharge. However, the results of C dating of groundwater sample in deep well were greater than 6400, and the concentration of tritium in water samples were close to 0 TU, meaning that the deep groundwater extracted at present was not made by modern rainfall replenishment. This finding differed from the previous studies of groundwater hydrogeology in the study area.