The phase transition of the Gauss-Bonnet AdS black hole has the similar property with the van der Waals system, and it is determined by the Gauss-Bonnet coefficient {\alpha} instead of the pure one between a big black hole and a small black hole. With this issue, we introduce a new order parameter to investigate the critical phenomenon of Gauss-Bonnet AdS black hole by the Landau continuous phase transition theory. Furthermore the critical exponent has been given. The recently works are mainly on some properties of the black hole microstructure by assuming that black holes are made up of effective black hole molecules and introducing the density of black hole molecules and the phrase transition. For the Gauss-Bonnet AdS black hole, the coefficient {\alpha} plays a key role in the Gauss-Bonnet AdS black hole phase transition, which is not only determined by the horizon position. These results perhaps provide some help for people to deeply explore the black hole microscopic structure.