Calcification　is　a　critical　challenge　for　achieving　anaerobic　reactors＇　high-efficiency.　However,　the　aggregation　caused　by　calcification　at　both　granular　sludge　and　reactor　levels　remain　to　be　fully　understood.　Herein,　this　study　investigated　the　characteristics　of　calcification　in　an　anaerobic　reactor　(RH)　operated　with　high　calciumcontaining　wastewater　for　over　200-day.　It　was　found　that　the　COD　removal　efficiency　in　RH　dropped　from　98.00　&　PLUSMN;　2.06%　to　41.29　&　PLUSMN;　3.79%,　which　was　much　lower　than　that　of　95.50　&　PLUSMN;　1.55%　in　the　control　reactor.　Morphological　analysis　revealed　that　the　high　influent　calcium　caused　granular　sludge　aggregation,　which　would　further　led　to　the　rapid　deterioration　in　bioavailability,　as　confirmed　by　both　mass　transfer　tests　and　theoretical　simulations.　Moving　forward,　statistical　analysis　showed　that　the　proportion　of　bioavailability　deterioration　zones　in　RH　system　(61.68%)　was　similar　to　the　decreased　COD　removal　efficiency　(57.87%),　proving　that　bioavailability　deterioration　was　the　culprit　for　the　performance　decline　of　anaerobic　reactor.