Photocatalytic oxidation techniques are promising for degradation of the highly ecotoxic and refractory isothiazolinone bactericides in relevant industrial wastewaters. However, low charge separation and directional transport efficiency under solar light radiation restrain their practical application. Here, we report a nanostructured photocatalyst doped with Gd and B in TiO₂ with carbon incorporation and defect formation through incomplete calcination. The specific surface area, grain size, and hydrophilicity of TiO₂ are improved, which is beneficial for the interfacial reaction between the photocatalyst and pollutants. The reduction of the bandgap, the broadening of the photo-absorption range, and the retarded electron–hole recombination promote the photocatalytic performance due to the improved oxygen vacancies based on the electron distribution modification. The difference in partial density of states (ΔPDOS) between the current catalyst and raw TiO₂ indicates that the co-doping of Gd and B with incomplete calcination changes the electronic hybridization of conduction band and valence band near the Fermi level, and affects the band gap energy. It improved charge separation and directional transport efficiency and benefited the formation of main active species, including ^•OH and O₂^•−, for the pollutant decomposition. The rate of photocatalytic removal of benzisothiazolinone (BIT) by the current photocatalyst reaches 1.25 h⁻¹, being 4.31 times that of TiO₂. The current work offers a constructive approach to the design and synthesis of nanostructured photocatalysts for the photocatalytic degradation of refractory organic pollutants.