The invention discloses a polymer nano material preparation method. The method comprises the following steps: adding a polymer in a first organic solvent, adding acetanilide, heating the materials and stirring the materials to a uniform state, dumping the material in distilled water for precipitating white crystal, uniformly stirring the material, performing pumping filtration, drying the material; grinding the dried white crystal, adding a second organic solvent, performing ultrasonic dispersion, using an organic microfiltration membrane for pumping filtration, and drying the material to obtain the polymer nano material. The polymer and acetanilide are dissolved in the organic solvent, and then the materials are dumped into the distilled water for precipitating the white crystal. The characteristic of acetanilide dissolved in water (0.46 g/100g, 20 DEG C) and easily dissolved in ethanol (36.9 g/100g, 20 DEG C) is used, acetanilide in the white crystal can be removed by ethanol, and the polymer nano material is obtained.