Alcalase2.4 L was selected for the hydrolysis of protein in decalcified crab shell.The optimum conditions for hydrolysis,microstructure of chitin deproteinized and the amino acid composition in hydrolysate were investigated.The degree of hydrolysis was up to about 14% when the hydrolysis was carried out at the optimal conditions(60℃,pH8.0,solid to solution ratio 1∶ 3 and enzyme to substrate ratio 3000U/g).The surface of chitin deproteinized by Alcalase2.4 L was glabrous and it was different from the microstructure of chitin deproteinized by alkaline solution.The amino acid composition in hydrolysate obtained by alcalase2.4 L hydrolysis corresponded with the ideal demand mode of human that is recommended by FAO and WHO.The content of umami amino acids was high in the hydrolysate.