The objective of the present paper was to study the effect of monomethoxypolyethlene glycol (mPEG) modification on the stability of chicken IgY and compare the stability of the modification products by Fourier transform infrared spectroscopy (FTIR), CD spectrooscopy and fluorescence spectroscopy. NHS-mPEG was used to modify IgY after mPEG was activated with N-hydroxysuccinimide (NHS). The optimal reaction condition for modification was 1∶10 molar rate of IgY to mPEG at pH 7, reaction for 1 h, and the product was obtained with modification rate of 20.56% and activity reservation of 87.62%. In addition, the thermal and pH stability of IgY and mPEG-IgY was compared by spectroscopic methods. The results showed that the α-helix, β-sheet, β-turn, and random content of IgY changed from 14.5%, 42.1%, 6.2% and 37.2% to 1.6%, 55.25%, 5.8% and 37.5%, while mPEG changed from 12.9%, 42.7%, 6.3% and 38.1% to 3.1%, 50.5%, 7.2% and 39.2%, respectively, after incubating for 120 min at 70 ℃. For the treatment with acid-base, similarly, the structure changes of mPEG-IgY were smaller than IgY. Thus, it is indicated that IgY modified by mPEG had greater stable properties.