The invention discloses a method for preparing 5-cyano iminostilbene, comprising the following steps that: carbamazepine shown in formula (II) and bis (trichloromethyl) carbonate shown in formula (III) are taken as raw materials which react in an organic solvent for 1 to 30 hours at a temperature of between 20 and 150 DEG C, the reaction liquid is processed, the 5-cyano iminostilbene (I) is obtained, and the reaction equation is shown below. The method uses bis (trichloromethyl) carbonate to replace P2O5 or POCl3 or SOCl2 as a dehydrating reagent to prepare the 5-cyano iminostilbene, thereby greatly reducing the safety and environment protection hidden trouble at the source; meanwhile, the method has simple and reasonable process, and high yield of the product and is suitable for industrial production.