The invention discloses a method for synthesizing Coumestan derivatives (I). In the method, a 3-aryl- 4-hydroxycoumarin compound (II) is dissolved in a non-protonic solvent, and under the action of an anhydrous ferric halide (III), the sp2 carbons of aromatic rings are directly bonded with branch oxygen atoms to form the Coumestan derivatives (I), wherein the R1 represents methyl, methoxyl or hydrogen atom; and R2 represents methyl, bromine atom, methoxyl or hydrogen atom. The method has the advantages of simple operation, cheap and easily available raw materials, ideal yield, low cost and the like.