The invention belongs to the technical field of isocyanate preparation and relates to a method for preparing isocyanate in the presence of an inert solvent. The method comprises that an inert solvent and amine are mixed and undergo a reaction with phosgene in a reactor to produce a gas mixture, the gas mixture flows into a chiller and is transformed into a gas-liquid mixture, a chilling agent is sprayed into the chiller, the gas-liquid mixture flows out of an outlet of the chiller and enters into a storage tank, a storage tank gas phase in the storage tank directly enters into a washing tower, and a storage tank liquid phase as an isocyanate product is fed into a rectification refining unit. The method has simple processes, is convenient for operation and has a high energy utilization rate and a low cost. The chiller has a long service life. In chilling, aerosol is not produced. The prepared isocyanate has less impurities and high purity.