Most of the ion exchange membranes distinguished monovalent ions from the multivalent ions according to the charge difference, but scarcely had selectivity to the specific ions. In order to effectively separate potassium ions from seawater containing more than 30 times sodium ions, potassium ion-sieve composite membranes with specific selectivity to potassium ions had been synthesized in this paper. The potassium ion sieve was successfully synthesized on the flat alpha-Al2O3 membrane by secondary hydrothermal method to form potassium ion sieve composite membrane. Then, the results of SEM, XRD and EDS showed that the potassium ion sieve had been successfully bonded with base membrane. The results of electrodialysis experiments showed that the ionic permselectivity were alpha (K/Na) = 7.62, alpha (K/Mg) = 68.43 and alpha (K/Ca) = 162.92 respectively, which indicated that the composite membrane had special selectivity for potassium ions. The composite membrane had been evaluated under different voltages and temperatures. As the voltage increased, the ion flux and the perm-selectivity to potassium were all improved. Moreover, with the increase of temperature, the ion flux increased but the selectivity to potassium decreased. The separation performance of the composite membrane was studied by using equimolar solution (containing K+, Na+, Ca2+, Mg2+), seawater and twice concentrated seawater as the test solutions, and the results had exhibited that the composite membrane had good application performance in extracting potassium from seawater.