The invention discloses a waste water treatment system. The system includes a coagulation reactor and an anaerobic reactor which are in successive connection along a direction of a waste water treatment process; the coagulation reactor has a waste water inlet; the anaerobic reactor has an anaerobic water outlet; and the anaerobic water outlet of the anaerobic reactor connected to the waste water inlet of the coagulation reactor through a return line. According to the embodiments, the system has advantages of being good in COD treatment effect, not easy to scale and low in operation cost.