The　recovery　of　phosphorus　(P)　from　sewage　sludge　through　pyrolysis　is　one　of　the　most　promising　alternatives　to　alleviate　the　shortage　of　P　supply　in　the　future.　Adding　calcium　additives　is　beneficial　to　the　conversion　of　available　P　and　helps　to　improve　the　recovery　and　utilization　efficiency　of　P　in　biochar.　This　study　thoroughly　examined　the　influence　of　CaCO3　on　the　speciation　and　slow-release　behavior　of　P　in　sludge　during　pyrolysis　at　500　and　700　degrees　C.　The　addition　of　CaCO3　could　enhance　the　conversion　of　non-apatite　inorganic　phosphorus　(NAIP)　into　more　accessible　apatite　phosphorus　(AP).　We　observed　that　adding　CaCO3　(2.5-20　%)　to　raw　sludge　at　500　degrees　C　increased　the　proportion　of　AP　in　inorganic　phosphorus　(IP)　from　65.2　to　97.3　%　but　did　not　increase　the　concentration　of　AP,　which　remained　between　31.03　and　31.64　mg/g.　At　700　degrees　C,　introducing　CaCO3　(2.5-20　%)　boosted　the　AP　content　from　35.08　to　41.38　mg/g　and　increased　the　proportion　of　AP　in　IP　from　67.7　to　98.6　%.　Moreover,　irrespective　of　the　pyrolysis　temperature　being　500　degrees　C　or　700　degrees　C,　the　final　main　form　of　P　in　biochar　with　CaCO3　was　Ca5(PO4)3OH,　according　to　experimental　characterization　results　(X-ray　photoelectron　spectroscopy　and　X-ray　diffraction)　and　thermochemical　calculations.　Additionally,　the　efficacy　of　citric　acid　(CA)　in　releasing　biochar-bearing　P　was　tested　in　a　laboratory　extraction　experiment　for　21　days.　CaCO3-　amended　biochar　showed　a　significantly　slower　total　phosphorus　(TP)　release　than　the　biochar　without　CaCO3　daily,　and　the　amount　of　TP　decreased　with　increasing　amounts　of　added　CaCO3.　The　finding　of　this　study　in-dicates　that　biochar　generated　by　adding　10　%　CaCO3　has　the　highest　AP　concentration　(41.38　mg/g)　at　700　degrees　C,　and　its　TP-release　efficiency　in　CA　is　0.29-1.98　mg/g　in　21　days,　which　shows　remarkable　potential　as　a　soil　fertilizer　with　slow　P　release.