The invention discloses a method for preparing isocyanate. The method comprises the following steps of preheating primary amine, phosgene and inert gas to a temperature which is 5 to 100 DEG C higher than the boiling point of primary amine respectively, mixing the preheated primary amine with the inert gas so as to obtain primary amine mixed gas, mixing the primary amine mixed gas with the phosgene in a mixer and performing primary reaction so as to obtain a primary reaction mixture, enabling the primary reaction mixture to enter a tubular reactor for further reaction so as to obtain gaseous isocyanate, and separating and refining the gaseous isocyanate so as to obtain the isocyanate. The preparation method avoids using organic solvent diluents, reduces the energy consumption for gasifying and recovering the organic solvent diluents, and saves cost.