In this paper, we report a new combined method of chemical coprecipitation-ionic liquid electrodeposition for the determination of trace uranium in seawater. The procedure involves coprecipitation of trace uranium on Fe(OH)3 upon addition of ferric chloride and electrodeposition of uranium in EMIMBF4 to determine by inductively coupled plasma optical emission spectrometry(ICP-OES). The solution, which ferric chloride was added to 1 L of seawater sample and adjusted the pH to 4, was placed at 80 °C for 30 min to coprecipitate uranium with iron hydroxide. The precipitate was then dissolved in nitric acid and electrodeposited in EMIMBF4 to determine uranium by ICP-OES. The coefficient of diffusion (D) of electrodeposition of U (VI) on platinum electrode in EMIMBF4 was evaluated, which was 3.31 × 10−9 cm2/s. The electrochemistry experiments indicated that the reduction of U (VI) at platinum electrode in EMIMBF4 was a quasi-reversible single step two-electron transfer. The percent of chemical coprecipitation recovery of U (VI) was 98.9% and recovery in electrodeposition was 99.5%. Using combine of two methods can determined trace uranium in seawater.