This article describes the application of geologic analysis and combinative economic analysis to select the optimal site for a LNG plant in Iran. Collecting the geologic data of the intended site A & B, combined with field geological survey, we extract the geologic features of the site to be selected, and know that the sand, calcareous cementation and submerged reef are key influential factors for the flat land site, foundation treatment, pile foundation and pile driving process, channel excavation, construction of the main terminal, power plant and liquefaction plant base, and are the core influencing the engineering cost. The conclusion is drawn that site B is the best one .