To investigate the effect of the flue gas desulfurization gypsum (FGDG) on coastal cultivated soil, a field experiment was conducted to evaluate different application amounts at rates of 0, 15, 30, and 45 t/hm². The effect of FGDG on soil total phosphorus (P), available P, and inorganic P fractions in coastal cultivated soils were measured. The results indicated that FGDG had no significant effect on soil total P in the cultivated soils. However, FGDG significantly reduced the soil available P and tile flow water soluble P. The inorganic P content was significantly increased by FGDG, especially calcium phosphorus (Ca₈-P). The mean concentration of Ca₈-P in the soil increased by 35.2%-66.3%, and the Ca₁₀-P concentration in the soil was increased by 7.3%-17.8%. The ability of Echinacea to absorb P was not affected by the reduction of available P or increased Ca-P in fractions of inorganic P in the soil. FGDG application promoted plant growth and development. Therefore, FGDG could effectively fix the soil available P, control the excessive P accumulation of soil, and reduce the risk of eutrophication of the surrounding water bodies. The results provide a guideline for a reasonable rate of FGDG application in coastal cultivated soils.