The present invention discloses a novel use of the FGD byproduct from thermal power plants and facilities and the specific method for alkali soil amelioration. In this method, after sampling, amount of Na.sub.2CO.sub.3, NaHCO.sub.3, Mg(HCO.sub.3).sub.2,.+-.)Na and .+-.)Mg can be calculated by analysis. According to the component in FGD byproduct from thermal power plants and facilities, the amount for FGD byproduct from thermal power plants and facilities needed in the alkali soil to be ameliorated can be determined. The FGD byproduct from thermal power plants and facilities are scattered on the surface according to the alkalization degree. After irrigating, the soil is operated just like common land. This invention allows the FGD byproduct from thermal power plants and facilities to be utilized effectively and economically, changing from it from waste to a useful substance. Moreover, it provides a method for alkali soil amelioration, which is fast, efficient, consumes less water, is cost effective, and promising.