The idea of varying volume ratio of water to ethanol in solvent was firstly employed to yield phase composition controllable mixed-phase titanium dioxide (TiO2) photocatalysts via a low temperature solvothermal route at 353 K. It was found that anatase contentsincrease from 0 to 100% with increase of ethanol contents in solvent. The mixed-phase TiO2 with 60% anatase content exhibited thebest photocatalytic activity in photodecomposing formaldehyde (FAD) under UV light irradiation, which increases by about 270 or400% in comparison with single futile or anatase TiO2, and is slightly better than commercial mixed-phase TiO2 (P25). The synergisticeffect in mixed-phase TiO2 including antenna effect by rutile phase and the activation effect by anatase phase was observed. Further,partially quantitive correlation between phase composition and photocatalytic activity was ascribed to maximizing the synergistic effectand an optimal value of anatase content about 60% in mixed-phase TiO2 photocatalyst was therefore suggested.