The wide use of uranium in human activities lead to a wide distribution of uranium compounds,attracting great attention to the study of uranium complexation with small molecule ligands such as organic acids.In the present work,the binding constant of uranyl-citrate complex was determined by affinity capillary electrophoresis using a multi-injection method to measure the electrophoretic mobility.The background electrolytes used contained HC104 and NaC104,with a pH of 2.0 and ionic strength of 0.050 mol/L,then citric acid was added to reach different concentrations.The binding constant was calculated by nonlinear fitting assuming that the reaction had an l:1 stoichiometry and the complex was [(U02)(Cit)]-.The obtained binding constant by the multi-injection method is lg K = 9.68.