Abstract: The thick source method is widely used in the detection of gross α/β radioactivity in water bodies due to its low detection limit. However, this method has the problems of many chemical reagents, complicated and time-consuming manual operation, which can’t report the radioactivity index of water bodies in time. In order to reduce the manual operation and improve the detection efficiency, the method of gross α/β radioactivity detection in water based on membrane enrichment technology was proposed. The principle of this method is to give a certain pressure for water to pass through the reverse osmosis membrane to retain the solutes in water on the membrane surface, and then measure its gross radioactivity after drying and curing, which has the characteristics of simple operation and no chemical reagents. And the single test process was simplified into several steps including new membrane weighing, background measurement, filter enrichment, wet membrane drying, dry film weighing and activity measurement. Thanking to this simple process, an automatic detection device for gross α/β radioactivity in water was developed and manufactured, and the enrichment recovery of α and β nuclides by the membrane modules under this system was studied and determined. The research and development of reverse osmosis membrane components and the basic structure of automatic radiation detection device for detecting gross α/β radioactivity in water were introduced in this paper. The functional relationship between powder source quality and detection efficiency was studied, with fitting coefficients of higher than 0.99. In addition, the concentration recoveries (P) of α and β nuclides were studied and determined. The results show that the recoveries of α and β nuclides in the system are higher than 99% when the volume of water sample is 2 L. In real environmental samples, the relative errors of the gross α radioactivity measured by the new method were 3.94% and 5.17%, the relative errors of the gross β radioactivity measured by the new method were 2.04% and 11.93%, compared with those measured by two other institutions using the thick source method. The self-developed detection device was used for continuous automatic detection in the external field for a month. The results show that the system operates stably, and the sample preparation time is shortened by 77%, the detection cycle of a single sample is shortened by 70%, and the relative error of the gross α/β radioactivity detection results is less than 10% compared with the thick source method. In conclusion, the new method is in good agreement with the thick-source method and achievs a balance between accuracy and timeliness. Moreover, the detection time is greatly shortened.