The invention relates to nano-TiO2 aqueous dispersion liquid and a preparation method and application of the nano-TiO2 aqueous dispersion liquid. The method comprises the following steps: (1) mixing the aqueous solution of a titanium source with the aqueous solution of a precipitant, and reacting for more than 2 h at the temperature of 60 to 110 DEG C to obtain sol; (2) adjusting the pH value of the sol to be 5 to 10 to obtain white suspension liquid; and (3) dispersing the white suspension liquid and then adding a stabilizer to be mixed to obtain the nano-TiO2 aqueous dispersion liquid. The nano-TiO2 aqueous dispersion liquid prepared by adopting a low-temperature sol method is high in dispersibility and stability and capable of forming a TiO2 film on a substrate, has higher photocatalytic activity in sunlight and ultraviolet light, and for example, the photocatalytic degradation rate of rhodamine B in water is up to 90%, and further is simple in operation, mild in reaction conditions, low in energy consumption and easy in popularization and application.