By using the density functional theory, six structures of oligopeptides chain configuration, consisting of glycine and tryptophan alternatively, are optimized at the B3LYP/6-31 G(d) level. The average binding energy and IR spectrum are calculated. Results show that the stability of oligopeptides grows monotonously with the peptide chain growth. The vibration infrared spectrum analysis show that with the growth of oligopeptides peptide chain, the vibration frequency of one functional group shows blue shift or even-odd shift; while coupling vibration of the same functional group shows red shift, and steady frequencies at the end of chain appear on the infrared spectrum, that is to say, coupling effect, parity effect and size effect exist when, glycine tryptophan oligopeptides, consisting of glycine and tryptophan alternatively, change with peptide chain. The result is significant in measuring the length and number of residue of peptide chain.