The galacto-oligosaccharide was synthesized continuously by immobilized Bacillus stearothermophilu producing beta-galactosidase in fibrous bed reactor. The effect of substrate concentration, pH, reaction temperature and retention time on production of GOS was investigated. The optimal reaction conditions were determined. Substrate concentration were 450 g/L; Reaction temperature was 55 degrees C; pH was 7.0; Residence time was 100 min. The product yield reached up to 50.7%. GOS synthesis was promoted by feeding 1.5% D-glactose after 24 h. The immobilized cell reactor can work stably for 120 h.