S-adenosylmethionine(SAM) synthetase was immobilzed on sodium alginate-gelatin and then cross－linked with glutaraldehyde for improving the stability of the immobilized enzyme. Properties of the immobilized enzyme were identified. The results showed the optimal conditions for the immobilization of the enzyme were as follows: the mass fraction of sodium alginate, gelatin and calcium chloride was 2.0%, 1.0% and 4.0% respectively; the amount of enzyme was 2.5 mg/mL gel; the volume fraction of glutaraldehyde was 0.6%. The cross-linked immobilized enzyme showed a good stability compared with the free enzyme. After incubation at 50℃ for 5 h the immobilized enzyme maintained 72％ of the original activity while the free enzyme lost all activity. The cross-linked immobilized enzyme showed a good stability in alkaline solution. It still kept more than 87％ of the original activity when incubated in the buffer of pH 8.0～pH9.0 on 4 ℃ for 10 h. The cross-linked immobilized enzyme was employed to synthesize the SAM and it remained 65% activity after eight times repeated operations.