The invention discloses a method for synthesizing n-butyl isocyanate by adopting a vapor phase method. The method comprises the following steps: firstly, preheating a raw material n-butylamine; preheating the raw material phosgene; introducing the preheated gaseous n-butylamine and preheated phosgene into a fixed bed reactor containing a catalyst to react, and condensing the materials, removing phosgene and rectifying the reacted material to prepare an FNC finished product. According to the method, the n-butyl isocyanate is continuously synthesized by adopting a gas-phase solvent-free method, the reaction time is greatly shortened, the process route is short, and the method is suitable for industrial production, wherein the FNC is catalytically synthesized by adopting a fixed bed, the reaction is carried out under a pressurized condition, and the reaction conversion rate and selectivity are relatively high; in addition, the FNC is synthesized by a solvent-free gas-phase one-step method, the post-treatment process is simple, the procedures of desolventizing, recovering the solvent and the like are not needed, the energy consumption is greatly reduced, and the cost is remarkably reduced.