The synthesis of nano-TiO2 materials have attracted intense interest due to their importance in a wide area of applications. In this study, we report a facile method to synthesize mixed-phase TiO2 nanocomposites by using a one-step approach under mild solvothermal conditions. Differently from previous reports, this method not only yields rutile/brookite/anatase TiO2 nanocomposites with high photocatalytic activities, but also can obtain highly oriented single-crystal rutile nanorod arrays selectively deposited on FTO. These products were characterized by XRD, FTIR, FESEM, TEM, HRTEM, and BET. Results indicate that in the brookite/anatase/rutile coexisting nanopowders, the brookite and anatase phases were crystallized into irregular nanoparticles <20 nm in diameter, whereas the rutile phase was crystallized into single-crystalline nanorods ∼20 nm in diameter and 100 to 500 nm in length. The single-crystalline rutile nanorods could form a film with controllable thickness up to ∼7 μm. The sample with 29.9% anatase, 27.9% brookite, 42.2% rutile was shown to have the highest photocatalytic activity, yielding over 90% bleaching of methyl orange solution in 20 min. The degradation rate constant k of this sample was 0.10180 min−1, almost twice as high as that of P25 (k = 0.05397 min−1). DFT calculations were used to confirm the band structures and density of states in brookite, anatase, and rutile phases.