The present invention discloses a sewage adsorbent and a preparation method thereof, wherein the main raw materials of the sewage adsorbent comprise, by weight, 25-35 parts of sodium-based bentonite, 12-20 parts of polyacrylamide, 5-8 parts of polyvinylpyrrolidone, 2-8 parts of sorbitan trioleate, 17-24 parts of nanometer modified active carbon, 0.7-1.3 parts of a polyoxyethylene ether-based surfactant, 8-12 parts of polyaluminum chloride, and 0.8-2.0 parts of stearate, and the nanometer modified active carbon preparation method comprises: carrying out ultrasonic wave impregnation on active carbon by using nitric acid, magnesium acetate, polyurethane and sodium alginate, and drying to obtain the nanometer modified active carbon. According to the present invention, with the sewage adsorbent, the physical and chemical indexes of the seriously polluted sewage can achieve the GB18918-2002 standard; and the sewage adsorbent has advantages of simple treatment process, less consumption, good treatment effect, stable performance, high polluted-sewage recovery degree, effectively-reduced treatment agent cost, good economic benefits, and wide social benefits.