The formation of nanocrystals with a controlled composition and desirable nanostructure is a prominent challenge in many science and technology fields. In this study, rutile TiO₂ nanorods coexisting with anatase TiO₂ nanoparticles were fabricated via a mild one-pot hydrothermal process. Results indicate an interesting phenomenon wherein adding an appropriate amount of oxalic acid can realize proportional regulation of anatase and the rutile phase. An increasing quantity of oxalic acid changes the crystalline phase from rutile to anatase accompanied by morphological changes from TiO₂ nanorods to nanoparticles. The optimized anatase/rutile crystalline phase and TiO₂ nanorods/nanoparticles morphology enhance the light-scattering and electron transfer ability. When the percentage of anatase phase increased to 67%, the composite architecture grew by 60% in photoelectric conversion efficiency and 0.1 V in open-circuit voltage. This study opens a door to the fabrication of the controllable mixed crystal composition and the designed micro-nano structure for the improvement on the photovoltaic performance.