Transforming　municipal　sludge　(MS)　into　biochar　via　co-pyrolysis　with　calcium　salts　enhances　phosphorus　(P)　recovery　and　reduces　the　risk　of　heavy　metal　contamination.　In　this　study,　the　performance　and　mechanism　of　bioavailable　P　conversion　and　heavy　metal　stabilization　were　systematically　compared　under　co-pyrolysis　of　MS　with　CaCO3,　CaO,　and　CaCl2　at　mass　ratios　of　2.5-10　%　at　700　degrees　C.　The　results　showed　that　CaCl2　had　the　best　ability　to　convert　P　from　non-apatite　inorganic　P　to　apatite　P,　followed　by　CaO　and　then　CaCO3.　CaO　and　CaCO3　addition　induced　more　Ca5(PO4)3OH　formation,　while　CaCl2　induced　more　Ca2PO4Cl　and　Ca5(PO4)3Cl　formation.　The　degree　of　stabilization　of　heavy　metals　(e.g.,　Cd,　Cr,　Cu,　Ni,　Pd,　and　Zn)　was　significantly　improved　after　co-pyrolysis　of　MS　with　these　salts,　thus　reducing　the　potential　environmental　risk.　In　particular,　because　of　the　dual　action　mechanism　of　chloride　ions　and　calcium　ions,　co-pyrolysis　with　CaCl2　exhibited　the　lowest　risk　of　heavy　metal　contamination　compared　with　CaCO3　and　CaO.　These　findings　suggest　that　addition　of　2.5　%　CaCl2　can　optimize　the　production　of　bioavailable　P　and　the　stabilization　of　heavy　metals　in　biochar.　This　treatment　is　recommended　for　the　safe　utilization　of　P　resources　derived　via　MS　pyrolysis.