In this rapid communication, we present the new electromagnetic resonant cavity properties of the gold nanoshell. The gold nanoshell is regarded as a nanometre electromagnetic resonant cavity, and the cavity resonance displays tunability controlled by the radii of the dielectric core. The main mode of nanocavity resonant absorption is obtained, which shows a redshift during the reaction of the gold nanoshells. The classical electrodynamics theory is used to describe this gold-coated mini nanoshell cavity, and it shows agreement with experimental results.