This paper describes a low-cost and fast method to evaluate gross α and β-radioactivities in natural water based on an online high-purity germanium detector gamma measurement system. The major gamma activities in nat-ural water are provided by natural and artificial radionu-clides such as 40 K, 137Cs, and radionuclides belonging to 238U and 232Th series. The main a emitters related to gamma emissions in natural water are 224Ra (240.98 keV) and 226Ra (186.21 keV), and the β- emitters are 40 K (1460.85 keV), 214Bi (609.31 keV), 208Tl (583.19 keV), and 214Pb (351.93 keV). The formula for gross α and β-activity concentration is based on these radionuclides, and the short half-life decay products are considered in the calculation. The detection efficiency of the device across energy region (0-3 MeV) is obtained through Monte Carlo simulation, and a calibration experiment is conducted to verify the simulation results. Gamma radioactivity is measured continuously for 114 d in Pixian County and Dongfeng Canal located in the Zouma River, Chengdu, Sichuan Province, China. A comparison of the calculation results and monitoring data from the Sichuan Management and Monitoring Center Station of Radioactive Environment indicates that the percentage and absolute error of a activity concentration is lower than 53% and 0.02 Bq/L, respectively, and that of β-activity concentration is lower than 33.2% and 0.016 Bq/L, respectively. The method can rapidly determine gross α and β-activity concentrations in natural water online.