The invention provides an automatic and rapid detection method and system for trace iron ions in power plant water vapor and application. The system comprises a reducing reagent bottle (1), a color reagent bottle (2), a buffer liquid bottle (3), a current-carrying bottle (4), a water sample bottle (5), a flow injection analyzer (9), a combination module (10), a Y-type tee joint (11), a color reaction disk tube (13), a reducing reaction disk tube (14) and a detection device (15), wherein the flow injection analyzer (9) comprises a main pump (6), an auxiliary pump (7), an eight-channel sampling valve (8) and a sampling ring (12). The invention further provides an automatic and rapid detection method for trace iron ions in power plant water vapor and application of the method in detection of trace iron ions in power plant water vapor. The method and the system provided by the invention can realize automatic, rapid, accurate and reliable determination of trace iron ions in power plant water vapor, and the detection limit is 0.5mu g/L.