Phosphorus　is　an　indispensable　element,　while　the　existing　phosphate　rock　resources　are　not　enough　to　maintain　the　long-term　demand　of　modern　agricultural　development.　To　ensure　the　sustainability　of　economic　and　social,　it　is　necessary　and　urgent　to　seek　the　alternative　phosphorus　source　of　phosphate　rock.　In　this　study,　calcined　clam　shell　powder　(CSP)　was　firstly　used　as　a　modifier　and　mixed　with　sewage　sludge　(SS)　for　co-pyrolysis　to　obtain　sludge-derived　pyrochars　containing　a　large　amount　of　bioavailable　phosphorus.　The　effect　of　different　temperatures　(500,　600,　700,　800　degrees　C)　and　dosages　(0%,　10%,　20%,　30%)　on　the　change　in　the　form　of　phosphorus　during　the　pyrolysis　process　were　investigated.　With　the　increase　of　temperature　and　the　addition　of　CSP,　the　orthophosphate　(Ortho-P)　increased　from　63.69%　in　the　original　SS　to　nearly　100%.　Furthermore,　the　main　phase　of　inorganic　phosphorus　(IP)　changed,　mainly　in　the　form　of　apatite　phosphorus　(hydroxyapatite　increased　from　8.88%　to　35.97%),　and　the　concentration　of　easy-to-lose　phosphorus　reduced　to　0.11　mg/g.　After　co-pyrolysis　of　SS　with　CSP,　the　passivation　situation　of　heavy　metals　(HMs),　such　as　Zn,　Mn,　Cr,　Cu,　Cd　and　Pb　was　significantly　improved,　thus　reducing　the　potential　environmental　risk.　The　planting　experiment　proved　that　the　sludge-derived　pyrochars　obtained　by　adding　20%　CSP　at　800　degrees　C　promoted　the　germination　of　seeds　and　the　growth　of　seedlings.　Consequently,　the　product　of　SS　co-pyrolysis　with　CSP　could　be　used　as　an　excellently　bioavailable　and　environmentally　friendly　phosphate　fertilizer　for　agricultural　production.　(C)　2021　Institution　of　Chemical　Engineers.　Published　by　Elsevier　B.V.　All　rights　reserved.