The process of anodic polarization of sulphide ion on static mercury drop electrode in NaOH solution includes not only the electrochemical reaction,but also the chemical reaction.So a film of "HgS" will be formed on the surfaoe of mercury electrode.Preliminary results indicated that the structure of first layer of the film is different from that of the two known HgS,i.e,black HgS(cub),and red HgS(heg).The formation of the film is related to the strong specific adsorption of sulphide ion.Therefore,all these factors should be considered when the cathodic stripping voltametry is used for quantitative determination of sulphide ion.