The preparation of NH4+-loaded merlinoite (N-zeolite) for extracting potassium was studied systematically in seawater. N-zeolite was gained from K+-loaded merlinoite (K-zeolite), in which potassium ions were exchanged by ammonium ions. The amount of reagents, hydrothermal reaction temperature and reaction time were detailedly researched in order to obtain the optimized preparation conditions of N-zeolite. The experiment showed that the ion exchange between ammonium ions and potassium ions could be instantly completed at room temperature. The ion exchange equilibrium and property of N-zeolite for potassium extraction were studied in seawater at room temperature. The results indicated the ion exchange capacity for potassium reached equilibrium in about 50 min. Since then, the maximum value of ion exchange capacity was 35.5 mg/g. The ion exchange separation factors for KK+/Na+, KK+/Ca2+, KK+/Mg2+ were 21, 91 and 247, respectively. N-zeolite had good properties of reproducibility and reutilization through six times of repeated modified experiments. K-zeolite and N-zeolite revealed a reversible type-III isotherm characteristic by N2 adsorption?desorption experiments. The separation mechanism of N-zeolite for extracting potassium selectively was dominated by the memory function for a specific ion (K+) as well as the similar ion size.