Indium is a silver-white rare and scattered metal with an average mass fraction of 0.000 01% in the earth’s crust. In order to accurately determine the low content of indium in the flue ash sample, we preliminarily analyzed the composition of the sample to ascertain the dissolved acid and its proportion in our work. The hydrochloric, nitric, hydrofluoric and perchloric acids (Ｖ∶Ｖ∶Ｖ∶Ｖ=15∶5∶2∶2) were successively and gradually added to completely dissolve the sample, and then cooled to room temperature and transferred into a shunt funnel. Following, the above-obtained mix was diluted with ammonium bromide as salt precipitator (the volume of solution was controlled to about 25 mL), and ethyl acetate as an extractant and diluents. The extraction solution was directly introduced into the inductively coupled plasma atomic emission spectrometry (ICP-AES) equipped with an organic injection system to determine the indium with the analytical spectral line 230.606 nm in the flue ash sample. Thus the method for determination of indium in the flue ash sample by ICP-AES was established. In this work, an extraction separation method was to eliminate the interference of matrix elements and coexisting elements after the sample was dissolved. The optimal experimental conditions were obtained through a series of conditional experiments on extraction acidity, extractant, extraction method, salt analyzer and analytical spectral line. The mass concentration of indium was linear with its emission intensity in the range of 0.25 to 4.00 mg·L-1 with the linear correlation coefficient 0.999 3 and the detection limit 0.03 mg·L-1. The relative standard deviation of the determination results (n=11) was less than 5%, and the recovery rate was 92%~102%. The determination results of indium in five flue ash samples were relatively consistent compared with inductively coupled plasma mass spectrometry (ICP-MS) method according to the above experimental steps. Moreover, this method would be used for batch detection of flue ash samples with indium content between 0.000 8% and 0.10% because of its simple, fast, sensitive, and higher accuracy advantages compared with existing analytical methods (EDTA titration, spectrophotometry, atomic absorption spectroscopy, X-ray fluorescence spectroscopy, inductively coupled plasma atomic emission spectroscopy, mass spectrometry, etc.).