To more accurately demonstrate how the electronic structure was changed by codoping TiO₂ with N–Al and to explain the origin of the enhanced photocatalytic activity by N–Al-codoped TiO₂, we investigated the electronic structures and microstructures of N monodoped, Al monodoped, and N–Al codoped anatase TiO₂ by theoretical calculation and experiments. It was found that N monodoping and Al monodoping could effectively narrow the band gap. The effect was doubled when TiO₂ was codoped with N and Al because the emergence of the N 2p state made the VBM move to a higher energy level and the emergence of the Al 3p state made the CBM move to a lower energy level in the system. Moreover, codoping of N and Al into TiO₂ did not change the microstructures of TiO₂ catalysts although it was proven that both N and Al atoms were codoped into TiO₂. N and Al-codoped TiO₂ displayed a higher crystallinity and a higher specific surface area than others. According to the results about the degradation rate and the dynamics of the catalysts, N–Al–TiO₂ exhibited the best photocatalytic activity; this suggested that N and Al were combined to enhance the photocatalytic activity of TiO₂.