The invention relates to a method for preparing water soluble nano titanium dioxide by using a low-temperature solvothermal method, which belongs to the technical field of nano inorganic compound nano titanium dioxide. The method mainly comprises the following steps of: alcohol discomposing, acid dissolving, water heating, washing, drying, and the like. During alcohol discomposing, polyethylene glycol (PEG) is introduced so as to ensure that nano titanium dioxide has good water solubility because the surface of the nano titanium dioxide is wrapped by the PEG after crystallization under a water heating condition. The method has simple process and conforms to the requirement of environmental protection. Proved by an ultraviolet light and visible light catalysis testing result, a TiO2 sample prepared by using the method has high photocatalysis activity. The water soluble nano titanium dioxide prepared by using the method can meet the application requirement on the dispersion of a water system.