The invention discloses a preparation method of micro-nano molybdenum carbide powder. The preparation method comprises the specific steps that an organic precursor serves as a carbon source, molybdate serves as a molybdenum source, fused salt serves as a reaction medium, high-temperature processing, washing and drying are conducted in sequence in an air environment, and the micro-nano molybdenum carbide powder is prepared. The preparation method is quick in reaction speed, and the reaction time is effectively shortened. Besides, the fused salt exists between the generated solid-phase molybdenum carbide particles in the reaction process, so that the mutual bonding between the particles can be stopped, and the agglomeration phenomenon can be reduced. In the method, no special atmospheric conditions are needed, the micro-nano molybdenum carbide material can be prepared at a relatively low temperature; the process is simple, the preparation conditions are mild, and a large scale of production is easy to achieve.