The invention discloses a preparation method for a tungsten trioxide nanowire electrochromic film, which includes the following steps: (Step 1) tungstic acid and hydrogen peroxide solution are mixed, and after water is added for dilution, stirring is carried out for aging, so that sol is obtained; (Step 2) the sol is applied on a conductive glass, and after annealing treatment, a conductive glass with a WO3 seed crystal layer is obtained; (Step 3) tungstate and water are mixed, pH is regulated to strong acidity, ammonium sulfate and alcohol are then added, and after stirring, tungsten precursor solution is obtained; (Step 4) the conductive glass with the WO3 seed crystal layer in the step 2 is immersed in the tungsten precursor solution in step 3 to carry out hydrothermal reaction, and thereby the tungsten trioxide nanowire electrochromic film is obtained. The prepared WO3 nanowire electrochromic film has an supramaximal optical modulation range and a high electrochromic rate; when applied as an electrochromic smart window, the tungsten trioxide nanowire electrochromic film can greatly reduce solar radiation, and can also play the role of saving energy and regulating temperature; moreover, the process is simple and controllable, the cost is low, and the preparation method is suitable for industrialized application.