Abstract: Enhancing the exposure ratio of high-energy 001 crystalline surfaces can effectively improve the photocatalytic activity of TiO₂, and a crystal surface control agent is usually used to expose the highenergy 001 crystalline surfaces, but the morphology of the synthesized TiO₂ is unstable and the particle size is large due to a variety of factors, so it is necessary to realize the stable synthesis of TiO₂ nanocrystals and reduce the size of the particles. In this paper, TiO₂ nanocrystals were synthesized using potassium titanate nanowires as precursors. Firstly, TiO₂ octahedral bipyramidal morphology was achieved through acid treatment, and the particle size of TiO₂ nanocrystals was reduced by decreasing the K+ content in potassium titanate nanowires. The study demonstrated that the photocatalytic activity of TiO₂-H7-0 sample was the strongest, which synthesized through acid treatment of potassium titanate nanowires for 7 hours, resulted in an 86% reduction in particle size compared to the untreated sample. Furthermore, the activity of degrading methylene blue was 7. 4 times higher than that of the original anatase TiO₂. In order to further improve the photocatalytic activity of TiO₂ nanocrystals, using（NH₄）₂CO₃ as a morphology control agent to control the exposure ratio of 001 crystalline surfaces through the variation of （NH₄）₂CO₃ concentration. The precursor without acid-treated series samples with（NH₄）₂CO₃ crystal facets modulation could effectively enhance the photocatalytic activity. The TiO₂-0. 14 synthesized at the（NH₄）₂CO₃ concentration of 0. 14 mmol/L exhibits 2. 6 times greater degradation activity than that of the original anatase TiO₂. However, the photocatalytic activity of the acid-treated for 7 h TiO₂ samples were suppressed by introducing（NH₄）₂CO₃ for crystal surface regulation, which might be caused by the lower percentage of 001 crystal surface exposure under the same（NH₄）₂CO₃ concentration, as well as the increase of the particle size with the increase in the concentration of（NH₄）₂CO₃, with the consequent decrease in the content of the oxygen vacancies. In this study, it was found that treating potassium titanate nanowire precursors with acid could synthesize octahedral bipyramidal TiO₂ nanocrystals and reduce its particle size, which could improve the photocatalytic activity of TiO₂. However, the extent of the acid treatment would impact the exposure ratio of the 001 facets and the crystallinity of the TiO₂ nanocrystals. The relationship between acid treatment and crystal facet modulation needs to be thoroughly explored to further enhance the photocatalytic activity of TiO₂ nanocrystals.