Abundant salts and complex constituents pose challenges to the enrichment of trace-level uranyl from field water samples. Based on the chemical behavior of the uranyl cation in several physical-chemical conditions (in the presence of MgCl2 etc.), we developed a facile and green strategy to improve the uranyl extraction from field water samples, different from the traditional way with organic impregnation. In our systems about nano-Mg(OH)2 treating salt lake brine, when the adsorbent is pretreated with trace amount of UO22+ ion, the ability to extract uranyl from brine can increase by 2-5 times. The molecular dynamics simulation and timeresolved laser-induced fluorescence spectroscopy analysis show that, in the pre-adsorption stage, UO22+ ions can steadily bind to Mg(OH)2 (001) surface because of the interaction with surface hydroxyl groups. When pre-treated Mg(OH)2 was exposed to brine, the coordination interaction between the pre-loaded UO2 2+ ion and the [UO2(CO3)x (2x-2)-] in solution engenders good selectivity and affinity for uranyl over competitive ions in brine. Besides, the coordination number of UO22+ by carbonate oxygen is large which further facilitates subsequent adsorption. Such strategy does not introduce other impurities, and can apply to metal-oxides-type adsorbents, e.g. TiO2. This study should shed light to further improvement of efficient uranyl extraction from field samples, and give insights into the mechanism understanding of uranyl adsorption in real systems.