Based on the problems of decentralized water supply and conventional ultrafiltration process, a gravity-driven membrane (GDM) filtration process was developed, which combined the dual rejections between bio-cake layer and ultrafiltration membrane and had the advantages of no cleaning, no chemical supply, simple operation, low energy consumption, and low maintenance. Results showed that flux stabilizations of GDM were observed during the long-term treatments of river water, reservoir water, and synthesized water, indicating the practical universality of GDM process in treating different types of water resources. The operating conditions, including raw water quality, operating pressure, membrane configuration type, membrane material, membrane pore size, temperature, intermittent filtration, and pre-treatment, affected the structures and compositions of bio-cake layer formed on the membrane surface, and consequently exerted impacts on the stable flux of GDM and its membrane fouling characteristics. Flux stabilization of GDM process was mainly determined by the structures and compositions of bio-cake layer regulated by the biological activities. The bio-cake layer with rougher structures, more abundant pores, and less extracellular polymeric substance (EPS) excretions, could contribute to a higher stable flux in long-term GDM filtration. Compared with conventional ultrafiltration process, the bio-cake layer formed on the surface of GDM could efficiently enhance the removal performance for turbidity, assimilable organic carbon (AOC), and ammonia. In addition, pre-treatment of slow filter could effectively modify the structural characteristics of bio-cake layer, improve the stable flux of GDM, and enhance the removal performance. The findings of this paper are expected to promote the extensive applications of ultrafiltration technology in decentralized drinking water supply.