According to the known thermostable mannanase ManAd3 sequence and the codon bias of yeast,the optimized mannanase ManA sequence( Accession No. KJ806637) was designed and synthesized. The ManA gene was cloned into the pPIC9 k vector,and then expressed in Pichia pastoris GS115 to obtain a recombinant strain ManA-GS115. The fermentation product of mannanase ManA was electrophoresis pure by SDS-PAGE identification with the molecular weight about 30 kDa. The enzymatic properties were shown that the optimal reaction temperature and pH value were 75℃ and pH6. 0 respectively,and the specific activity was up to 3 200 IU /mg. Moreover,the residual activity was above 90% after 30 min treatment under 75℃,pH 6. 0. With high expression,excellent thermostability,and the good thermostable tolerance under acidic condition,the mannanase ManA could be potentially applied in brewing,food,feed,textile and pharmaceuticals fields.