Tritium causes internal dose hazards to humans. Currently, tritium in the environment mainly comes from nuclear power plants, and tritiated water (HTO) is the main form of liquid emission. Therefore, online monitoring activity of tritium in the aqueous medium is vital for protecting human health, and it can warn of the operation status of nuclear power plants. In this paper, we present an improved structure of plastic scintillating fiber (PSF) and a PSF array design, and the detection parameters of PSF arrays with different radii and lengths under different radius detection chambers are simulated using Geant4. Based on the simulation results, the detector is designed, and the minimum detectable activity concentration (MDAC) of the detector is calculated. With calculated MDACs down to 3.09 Bq/L, the proposed design can meet the Chinese requirements for HTO release of inland nuclear power plants of <100 Bq∙L−1. Therefore, the detector designed with a PSF array can be applied to online monitoring of tritium in the aqueous medium.