Two acid partial hydrolysis methods are used in preparing three substrates, i. e. short-chain poly-mannuronide (SM), short-chain polyguluronide (SG) and short-chain polymannuronide-polyguluronide (SMG).Using viscosity reduction method, it is demonstrated that lyase Ⅰ is an exolyase while lyase Ⅱ and lyase Ⅲ are endolyases.Somogyi-Nelson method is used for determining the reducibility of lyase-decomposed substrates, indicating that the three lyases acted strongly on algin, SM and SMG, but slightly on SO. UV absorption method is used for determining absorption spectrum of products, indicating that all the three lyases are polymannuronide lyases (EC 4·2·99·4).With their aigins decomposed, all the three lyases can reduce the M/G value, the best being lyase I which can reduce the M/G by about 3.65 fola. Therefore, the enzymatic method might be used in preparing medicine for preventive absorption of 90Sr. Two acid partial hydrolysis methods are used in preparing three substrates, ie short-chain poly-mannuronide (SM), short-chain polyguluronide (SG) and short-chain polymannuronide-polyguluronide Lyase Ⅰ is an exolyase while lyase Ⅱ and lyase Ⅲ are endolyases. Somogyi-Nelson method is used for determining the reducibility of lyase-degraded substrates, indicating that the three lyases acted strongly on algin, SM and SMG, but slightly on SO. UV absorption method is used for determining absorption spectrum of products, indicating that all the three lyases are polymannuronide lyases (EC 4 · 2 · 99 · 4) .With their aigins decomposed, all the three lyases can reduce the M / G value, the best being lyase I which can reduce the M / G by about 3.65 fola. Therefore, the enzymatic method might be used in preparing medicine for preventive absorption of 90Sr.