The discovered proton halo nucleus of ²³Al is investigated in the nonlinear relativistic mean-field (RMF) model with deformation using the NL075 force parameter. It is shown that there is an energy inversion between the (5/2)⁺(202) and (1/2)⁺(211) orbitals in the ²³Al nucleus, which may produce a large enhancement of the reaction cross section compared with the neighbouring nuclei. Meanwhile, the NL075 force parameter may be better than the other RMF parameters for the calculation of the large deformed nucleus ²³Al.