A β-amylase gene from Clostridium thermosulfurogenes was cloned and heterologously expressed in Escherichia coli BL 21-SI.The influences of cell density at induction,temperature,and NaCl concentration were studied.The optimum condition of induction was at cell density of OD600,=0.6,temperature of 35 ℃,and NaCl concentration of 0.6M.The expression of recombinant β-amylase reached 118U/ mL in a 20L batch culture.In a 20L pH-Stat fed-batch culture,the cell density and enzyme activity were 2.35 times and 6.12 times of those obtained in batch culture.The soluble starch was converted to maltose by recombinant β-amylase and the conversion ratio was 62.2% after 3h reaction at 70℃.