According to bias in codon choice of Pichia pastoris ,The phytase phyA gene from Aspergillus niger N25 was mutated without changing its amino acid sequence. The expression plasmid pPIC9k-phyAm was constructed and transformed into GS115 strain. Positive clones,of which the chromosomes were integrated with phyA gene,were identified by the phenotype and PCR. SDS-PAGE analysis suggust that the size of enzyme protein of the expression product was about 70.15KD.Southern blotting analysis to the yeast transformants showed that phyA gene was intergrated into the chromosome genome. The phytase activity of PP-NPm-4-4 with codons optimized reached 136000Uoml-1 in malt wort culture medium after being induced with 36h, which was the 2.8 times of the original strain PP-NPm-8.