Underground brine formed in south coast of the Laizhou bay distributes widely with large volume since Late Pleistocene. Its origin is complicated. According to the analysis, it is the fluvial delta sedimentary system that creates a good replenishment, migration, filtration, capping condition for the generation of Underground brine. In the early stage of brine evolution, the lagoon water consisted of continuously injected normal seawater, which ran across the bar under the process of storm surge, and fresh groundwater lying below the river sediment. High salinity brine generated in lagoon, running into the tidal flat and the delta front belt under the effect of backflow—infiltration. Lagoon water would remain stable with relatively high concentrations for a long time. In the regression period, a large amount of long term evaporation and seasonal multi source river sediment transport modes formed brine producing reservoir. The terrestrial deposit covered early brine, which became Underground brine formations, and in this way, several large scale sea—land changes had formed alternative distribution characteristics. Taking the sand bar—lagoon as the brine generated reactor, dissolved components were fractionated under the effect of pump evaporation, and took ion exchange in sediments, which led to mineral alteration. As a result, Underground brine was generated in the tidal flat and delta frontier under the effect of backflow—infiltration.