The invention provides a method for preparing superfine spherical iron nitride powder, and belongs to the technical field of powder preparation. In the method, the superfine spherical iron nitride powder is synthesized and prepared by taking carbonyl iron liquid as a raw material, ammonia (or nitrogen) plasma as nitridation atmosphere and radio frequency (RF) plasma as a heat source for the reaction. The method comprises the following steps of: evaporating the carbonyl iron liquid; feeding the liquid steam into the nitrogen plasma by carrier gas; and reacting decomposed carbonyl iron steam with the nitrogen plasma to obtain the superfine spherical iron nitride powder. The method is characterized in that the nitridation and spheroidization of the carbonyl iron liquid steam are finished in one step by taking the radio frequency (RF) plasma as the heat source so as to prepare the superfine spherical iron nitride powder. The superfine spherical iron nitride powder produced by the method has fine and uniform particle size, high sphericity and high spheroidization rate; the process is simple; and magnetic fluid prepared by the method has high saturation magnetization.