The invention relates to a method for preparing indole through dehydrocyclizing o-ethylaniline. The method is characterized in that an RU-Al2O3 used as a catalyst boric acid adopted as a cocatalyst are added to a slurry bed reactor, nitrogen is introduced before a reaction to displace air, and o-ethylaniline is dehydrocyclized under a oxygen-free condition in order to synthesize indole. The method has the characteristics of low reaction temperature, easiness in operation and high indole selectivity.