Water-soluble F^- in coastal saline soil of Jiangsu Province changed firstly from low to high content, then from high to low during desalinization process of the soil. Water-soluble F^- content was controlled by water-soluble Ca⁺⁺ content(F^-=a+bCa^++-1/2)..Because of the mutual exchange reaction of OH^- and F^--ion within soil colloid and solution, the water-soluble F^- content increased and decreased reciprocally with the water-soluble Ca⁺⁺ content but simultaneously with the pH value. The mainfactor controlling the content of soluble fluorine might be the activity product of CaF₂. CaF₂ is the main solid phase controlling water-soluble F^- in the saline soils along the sea coast of Jiangsu.