The invention discloses a preparation method of ammonia-gas-free carbon-coated iron nitride nanopowder. The preparation method comprises the following steps of: uniformly dispersing iron nanopowder in a nitrogenous high polymer solution, cleaning the iron nanopowder by using water and then carrying out solidification treatment to obtain a composite precursor; then, carrying out oxidation treatment on the precursor or not, and then, continuously raising the temperature from room temperature to 500-800 DEG C at the temperature rise speed of 4-20 DEG C/min at a high-pure nitrogen or argon atmosphere to obtain the carbon-coated iron nitride nanopowder, wherein ammonia gas is not needed in the whole process. The reaction principle is as follows: the carbon-coated iron nitride nanopowder is produced through reaction between active nitrogen atoms released in the process of treating nitrogenous high polymers at the high temperature of 500-800 DEG C and iron nanopowder subjected to surface treatment. High-pure nitrogen or argon is adopted to replace ammonia gas in the treatment process, and long-term heat insulation is not needed, so that the production time is shortened, the cost is reduced, and the preparation method has the advantages of simple equipment and environment friendliness.