By potentiostatic polarisation for different time, passive films were formed on AISI 316SS. Characteris-tics of these films were studied using electrochemical measurement and surface ****ysis method. Theresults show the film thickness, film resistance and carriers’ density in the film all increase with time,and the increase of the film thickness and resistance both follows the direct logarithmic law. As timeprolongs, FeOOH penetrates deep into the film, and its concentration in the film also increases, while thedistribution of the other components of the film changes a little. The changing mechani**s of passivefilm characteristics with time were ****ysed.