To achieve real-time monitoring of gross beta radioactivity in drinking water, a scintillating fiber array system was developed. Using this system, the gross beta radioactivity in tap water at Tsinghua University campus in Beijing was monitored, and the same water samples were measured using the evaporation concentration method as a comparison experiment. Finally, the annual committed effective doses to children and adults who drank the tap water for a long time were estimated. The results showed that the gross beta radioactivity in tap water was 0.09 ± 0.03 Bq L−1 using the scintillating fiber array system and 0.076 ± 0.009 Bq L−1 using the evaporation concentration method. The annual committed effective dose values were less than the dose reference level of 0.1 mSv y−1, as suggested by the World Health Organization. The scintillating fiber array system can be used for measuring gross beta radioactivity in drinking water and protecting public health.