The invention discloses a synthesis process of 1-(2,6-dichlorophenyl)-2-indolone. The synthesis process comprises the following steps: (1) a reaction step: taking diclofenac as a raw material, adding an organic mixed solvent to dissolve the diclofenac completely, adding thionyl chloride at room temperature, and carrying out a reaction at proper temperature for a proper time, so as to obtain a mixture containing a 1-(2,6-dichlorophenyl)-2-indolone crude product;(2) a post-treatment step: filtering the mixture obtained in the step (1) to obtain a solid, washing the obtained solid with water, and drying to obtain the 1-(2,6-dichlorophenyl)-2-indolone product. The synthesis process has the advantages that thionyl chloride is taken as a reaction reagent, and the product yield reaches up to 90 to 95 percent, so that on one hand, the reaction cost is reduced, and on the other hand, the purification process is simpler and more convenient.