By using gene SOEing PCR, Arg (CGC) in the expression fragment of chitinase gene (chi58) from Chaetomium cupreum was mutated synonymously to Arg (AGA), which is a bias code of Pichia pastoris yeast. The expression plasmid pPIC9K-chi58A was constructed and transformed into GS115 strain through electroporation. The chitinase activity reached 101.71 U/mL±3.33 U/mL after induced with 120 h, which was the 3 fold of the original strain (31.83 U/mL±4.85 U/mL). Additionally, recombinant yeast showed good genetic stability after 10 cycle. SDS-PAGE analysis suggested that the weight of protein was 58 kD.