The traditional view is that the composition of sea water is invariable during the geological period. However, with the rapid development of the testing technology of fluid inclusion, more and more measured data of fluid inclusion reveal that the seawater composition occurred at least two cyclic changes during the Phanerozoic period. Based on measured data of primary halite fluid inclusions from multiple marine basins in the world, this paper discusses the paleoseawater geochemistry, water chemistry type, significant change of evaporite mineral assemblages since Phanerozoic period. Combined with the brine evaporation balance model of EQL/EVP, this study simulates quantitatively the evaporation process of two typical seawater, analyses the salt precipitation law and mineral assemblages characteristics during evolution process, and evaluates the prospect of potash deposits in marine sedimentary basin in China. The Spencer diagram reveals that the paleoseawaters composition occurs cyclical shift between Cl SO4 and Ca Cl phase zone during the past 600 Ma, the seawater type of Silurian, Devonian, Jurassic and Cretaceous is typical Ca Cl type, which is significantly different from modern seawater. The Mg 2k SO4 / Mg Ca 2k Jnecke diagram show that the halite inclusions composition of Silurian, Devonian, Jurassic and Cretaceous are located in Mg Ca 2K phase zone of CaCl2 rich and MgSO4 poor, Sylvite, carnallite stable phase zone, which indicate the original seawater has the feature of Ca rich and Mg poor in comparison with modern seawater, and the corresponding typical mineral assemblages are halite, sylvite, carnallite and tachyhydrite. But most of halite inclusions composition of Late Proterozoic, Permian and Tertiary, located in sylvite and kainite phase zone of Mg 2K SO4 diagram, is similar to modern seawater composition characterized by Mg rich and SO4 rich, and the typical mineral assemblages is halite, kainite, kieserite, sylvite and carnallitite. The quantitative model results of EQL/EVP show that the SO4 Mg rich and CaCl2 rich seawater have obviously different evaporation evolution path and mineral precipitation sequence, the CaCl2 rich paleoseawater is conducive to quality potash deposits, and the favorable metallogenic epoch is from Cambrian to Early carboniferous, and from Jurassic to early middle cenozoic.