This　study　aims　to　formulate　the　degradation　mechanism　of　polyamide　membrane　by　chlorine,　and　to　assess　the　role　of　Ca2+　or　Mg2+　involved　in　chlorination.　By　adjusting　chlorination　pH,　two　competing　degradation　mechanisms,　namely　chlorination-promoted　hydrogen　bond　cleavage　and　chlorination-promoted　hydrolysis,　were　first　time　proposed.　Hydrogen　bond　cleavage　promoted　severe　compaction　(reduced　pore　radius),　while　hydrolysis　led　to　a　loose　but　non-compactable　structure　(increased　pore　radius),　causing　opposite　trends　in　membrane　filtration　performance　at　different　pHs.　The　pore　radius　and　water　flux　were　reduced　by　33%　and　69%　at　chlorination　pH　4.0,　however,　water　flux　was　increased　by　45%　at　chlorination　pH　10.0.　Therefore,　intermolecular　rather　than　intramolecular　bonds　regulate　the　rotational　freedom　and　then　affect　compactness　of　polyamide　layers　under　pressure.　Ca2+　or　Mg2+　further　amplified　these　effects　of　chlorine,　i.e.,　water　flux　was　further　reduced　by　7%-10%　at　pH　4.0　and　further　increased　by　23%-48%　at　pH　7.0-10.0.　The　coordination　between　carbonyl　oxygen　and　Ca2+　or　Mg2+,　evidenced　by　simulated　molecular　electrostatic　potential　and　binding　energies,　initiated　excessive　hydrogen　bond　breakage　between　C--O　and　N-H.　Consequently,　it　prompted　N-chlorination,　as　non-hydrogen-bonded　N-H　has　a　higher　chlorination　priority　than　hydrogen-　bonded　N-H.　In　addition,　Ca2+　or　Mg2+　accelerated　chlorination-promoted　hydrolysis.