In　the　process　of　enhanced　coagulation　by　recycling　sludge,　drinking　water　precipitated　sludge　was　pre-treated　by　ultrasound　and　the　recycle　trails　were　subsequently　carried　out,　in　order　to　solve　the　enrichment　problem　of　poisonous　and　pernicious　substance.　The　effect　of　ultrasound　on　the　variability　offlocs　characteristics　of　drinking　water　precipitated　sludge　was　mainly　focused　on,　and　so　was　its　impact　on　coagulation　efficiency　of　sludge　recycling　process.　After　the　exposure　to　ultrasound,　the　temperature　of　sonicated　sludge　solution　was　raised　and　floc　size　was　decreased,　even　could　drop　to　a　few　microns　after　a　long-term　treatment.　And　The　floc　size　exhibited　a　linear　correlation　with　the　energy　density.　Zeta　potential　of　flocs　of　pre-sonicated　sludge　was　decreased　while　specific　surface　was　improved　with　energy　density　of　1　W/mL　and　5　W/mL,　contrastively　Zeta　potential　was　raised　and　specific　surface　was　decreased　with　energy　density　of　10　W/mL　and　15　W/mL.　The　coagulation　efficiency　was　enhanced　by　recycling　pre-sonicated　sludge　with　a　low　energy　density　(1　W/mL)　within　30　min　ultrasonication.　The　improvement　of　turbidity,　UV254　and　CODMn　removal　was　2.7%,　23.12%　and　10.02%　respectively.　Comparatively,　a　long　time　of　ultrasonication　under　high　energy　density　(15　W/mL)　could　reduce　the　coagulation　efficiency.　Therefore,　the　ultrasound　technique　could　enhance　the　pollutant　removal,　but　the　ultrasound　time　and　energy　density　must　be　well　controlled.