Abstract: The types of ion occurring in the field desorption mass spectra of Ⅰ A and Ⅶ A elements and inorganic salts are discussed. Spectra consist of a series of cluster ions (AB)_nA~+ (A-cation, Banion, n>0). The cluster ions is increasing with the radius of amions and cations decreasing. The periodical rule of element ionization energy can explain the FDMS characteristic of the kind of salts and predict FDMS characteristic of other group inorganic salts. According to analysis of known inorganic salt, the FDMS analysis of mixture of unknown inorganic and the mixture of inorganic and organic are introduced, as well as their unknow components are also confired.