Based on flow injection analysis, an automated analyzer for determination of iron in seawater was developed by on-line column preconcentration and chemiluminescence detection. The analyzer is composed of two stacked parts, a flow injection analyzer at the top and a Class-100 laminar flow chamber below for reagent solutions storage. The flow injection analyzer consists two peristaltic pumps, four valves and a packed photomutiplier tube (PMT). A mini column packed with commercial resin with iminodiacetic acid (IDA) group (Toyopearl AF-chelate-650M) was used for on-line preconcentration of iron from seawater matrix at pH 6.2. The iron was eluted from the column by 0.12 M Q-HCl solution and then merged with luminol solution just in front of the PMT window, where the light emitted by the solutions was detected. Under the typical setting with an initial seawater volume of 1.5 mL and a concentration factor of ~ 5, the detection limit reaches 0.05 nM, which suits most oceanic iron determination. The analyzer has been used during a 9-day long test cruise of GEOTRACES-China in March 2018 with a large sample throughput of over 100.