The expression of heterologous proteins may exert severe stress on the host cells at different levels. Protein folding and disulfide bond formation were identified as rate-limited steps in recombinant protein secretion in yeast cells. For the production of ${eta}$-glucosidase in Pichia pastoris, final ${eta}$-glucosidase activity reached 1,749 U/mL after fermentation optimization in a 3 L bioreactor, while the specific activity decreased from 620 to 467 U/mg, indicating a potential protein misfolding. To solve this problem, protein disulfide isomerase, a chaperone protein which may effectively regulate disulfide bond formation and protein folding, was co-expressed with ${eta}$-glucosidase. In the co-expression system, a ${eta}$-glucosidase production level of 2,553 U/mL was achieved and the specific activity of the enzyme reached 721 U/mg, which is 1.54 fold that of the control.