The invention discloses a method for preparing a nano titanium dioxide thin film on the surface of a dacron. The method provided by the invention comprises the following steps of: firstly, ultrasonically vibrating and washing the dacron to be treated for 10-30 min at a temperature in a range of 60-90 DEG C by utilizing treating fluid under the conditions that the frequency is 28 kHz and the poweris 100 w; then drying the dacron for future use; secondly, utilizing titanium sulfate solution and urea solution to subject the dacron to a hydro-thermal reaction; finally, washing the dacron by utilizing the washing solution in the bath ratio of 1 to 30-50 for 15-30 min at a temperature of 100 DEG C; repeatedly washing the dacron by hot water and cold water and then drying to prepare the nano titanium dioxide thin film on the surface of the dacron. The method for preparing the nano titanium dioxide thin film on the surface of the dacron provided by the invention saves raw materials, is convenient for operating and has a good use effect; and the washing resistance of the modified dacron is obviously improved.