The invention relates to a synthetic method of a coumarin derivative shown as a formula (III). The method comprises the following steps: under a nitrogen atmosphere, adding a compound shown as a formula (I), a compound shown as a formula (II), a catalyst, an accelerant and an auxiliary agent into an organic solvent; stirring at 70 DEG C to 90 DEG C and reacting for 5h to 8h; carrying out post-treatment to obtain the compound shown as the formula (III), wherein the formula (I), the formula (II) and the formula (III) are shown as the description, R1 and R2 are independently selected from H, C1 to C6 alkyl or halogen respectively, R3 and R4 are independently selected from C1 to C6 alkyl, unsubstituted phenyl or phenyl with a substituent group and the substituent group is C1 to C6 is alkoxyl or halogen. According to the method provided by the invention, the catalyst, the accelerant, the auxiliary agent and the organic solvent are comprehensively selected and cooperated, so that a target product can be obtained by a high yield and the method has wide industrial application value and production potential in the field of organic synthesis, especially in the field of synthesis of pharmaceutical intermediates.