A theoretical study on 13 oligopeptides of glycine and alanine by density function theory (DFT) is given in this paper. Geometric structures, vibration frequency, average binding energies, dipole moment are studied, and IR spectra of the oligopeptides are examined. Results show that with increasing number of residues the average binding energies tend to a regular pattern, IR frequencies of typical functional groups start to shift, but stretching and bending vibrations of the groups show the opposite trend of red shift and blue shift. It is revealed that the physical and chemical properties of the silk fibroin peptide chain show size effect and anisotropic in the growth process. This phenomenon may come from the coupling effect of the similar groups, and the different effects of intramolecular hydrogen bond on the stretching and bending vibration.