The invention discloses a processing method of selenium-containing waste water. The processing method comprises following steps: (1) pH value of waste water is adjusted to 2 to 6 with hydrochloric acid so as to realize reduction of a part of selenate radical ions (SeO4<2->) into selenite radical ions (SeO3<2->); (2) waste water processed via step (1) is subjected to cooling treatment; (3) polysaccharides are added into the cooled waste water for selenylation reaction so as to obtain selenopolysaccharides; and (4) the waste water processed via selenylation reaction is subjected to aerobic biological treatment. Selenium removing rate of the processing method is high; a large amount of selenium pollutants in waste water can be converted into selenopolysaccharides for recycling; COD and ammonia nitrogen in waste water can be removed at the same time with high efficiency; waste water high efficiency processing is realized; and economic benefits are improved greatly.