The invention discloses a method for treating slaughterhouse wastewater by adopting a CSTR-coagulation sedimentation-A/O technology. The method comprises the following steps: conveying the slaughterhouse wastewater into a water collecting tank, then conveying to CSTR and carrying out reaction, enabling effluent to automatically flow into a regulating tank, a coagulation sedimentation tank, an A/O reaction tank and a secondary sedimentation tank, carrying out mud-water separation in the secondary sedimentation tank, enabling supernatant to enter a disinfecting tank to be disinfected and discharged after reaching the standard, enabling partial sludge at the bottom of the sedimentation tank to reflux to an anoxic pond, discharging the rest of the sludge into a sludge concentration tank by virtue of a sludge pump, carrying out gravity concentration, dehydrating by virtue of a plate-and-frame filter press, and then transporting outward, so that treatment on the slaughterhouse wastewater is realized. By virtue of the method disclosed by the invention, the slaughterhouse wastewater can be comprehensively treated, so that content of CODcr, BOD, TSS and heavy metals in effluent obtained after the wastewater is treated is obviously lowered; meanwhile, coagulation reaction is carried out by virtue of a coagulator in the invention, heavy metals prepared from waste leather in a slaughter process are utilized for carrying out adsorption reaction, and the problem that the content ofnitrogen, phosphorus, heavy metals, feed organic matters and the like in low-concentration wastewater exceeds standard is well solved.