Taking Konjac powder as raw material, glucomannan oligosaccharides was prepared by hydrolyzing Konjac powder with immobilized β-mannanase. The experimental results showed that the reaction time, Konjac powder concentration, reaction temperature, enzyme amount and pH had different effects on the preparation of glucomannan oligosaccharides, in which the influence of concentration of Konjac powder and reaction time was larger than that of enzyme amount and pH. The optimal process conditions for the preparation of glucomannan oligosaccharides by hydrolyzing Konjac power with immobilized enzyme by orthogonal test were as follows: substrate concentration 1.5%, enzyme dosage 80×103 U/g, reaction time 6 h, reaction temperature 75 ℃, and pH 3.5. The yield of glucomannan oligosaccharide was 29.5%.