The invention discloses a post-sludge reduction method for treating urban sludge through titanium-white waste acid. The post-sludge reduction method includes the steps that sedimentation sludge is subjected to oxidation pretreatment through a fenton reagent formed through the titanium white waste acid to achieve the aim that the whole sludge discharging amount is reduced. By means of the post-sludge reduction method, pollution of discharging of the titanium-white waste acid on the environment is solved, the sludge amount generated in the wastewater treating process is reduced, and the effect of using waste to treat waste is achieved. The post-sludge reduction method is short in time consumption, easy and convenient to operate, low in cost and universally suitable for residual sludge weight reduction treatment.