The electrochromic PANI film was prepared by emulsion polymerization with dodecyl benzene sulphonic acid (DBSA) as dopant and ammonium persulfate (APS) as initiator. Ultrasonic dispersion was adopted in the polymerization. The electrochemical properties, the surface morphology and structure of the prepared PANI film was characterized by means of Fourier Transform infrared spectroscopy (FT-IR), cyclic voltammograms (CV) and field emission scanning electron microscope (FE-SEM), respectively. The relationship between the morphology and properties of PANI film was detailedly discussed. The PANI film exhibited an excellent electrochromism with reversible color changes form yellow to purple. The PANI film also had quite good reaction kinetics with fast switching speed, and the response time for oxidation and reduction were 65 ms and 66 ms, respectively.