The invention relates to a preparation method of indoxacarb intermediate. The preparation method is characterized by comprising the following steps: dissolving intermediate 5-chloro-2-methoxycarbonyl-2-hydroxy-1-indanone into xylene, adding benzyl carbazate and phosphorus pentoxide, separating out phosphoric acid, adding diethoxymethane into reaction liquid, enabling the solvent xylene to flow back on a tower, dropping the obtained reaction liquid into xylene at a constant speed, gathering ethyl alcohol, performing negative pressure desolventizing on obtained reaction liquid, adding a crystallizing agent into the rest liquid, performing cooling crystallization, filtering, and drying, so as to obtain 7-chloroindeno [1,2-e][1,3,4] oxadiazine-2,4a (3H,5H)-dicarboxylicacid-4a-methyl-2-benzylester. The method is convenient to operate, increases the reaction temperature, accelerates the reaction speed, and obviously improves the content and the yield of the intermediate.