The invention provides a method for evaluating grade of flue gas desulfurization gypsum. The method comprises the following steps: 1) contents of sulfur dioxide in a flue gas desulfurization gypsum sample and magnesium salt and chlorine salt in water-soluble substances are measured, wherein the magnesium salt is represented in the form of magnesium oxide and the chlorine salt is represented in the form of chloride ion; 2) crystal water KW content in composition of the flue gas desulfurization gypsum sample is measured; 3) crystal water content of impurities including calcium sulfite hemihydrate, MgCl2.6H2O and MgSO4.6H2O existing in the flue gas desulfurization gypsum sample is calculated according to the result measured in the step 1), content of crystal water in calcium sulphate dihydrate is obtained by subtracting the content of crystal water in the impurities from the content of crystal water of the flue gas desulfurization gypsum sample that is measured in the step 2), and grade of flue gas desulfurization gypsum is evaluated according to the content of crystal water in calcium sulphate dihydrate. In comparison with the prior art, the method provided by the invention can be adopted to more accurately evaluate grade of flue gas desulfurization gypsum.