The invention discloses a preparation method of a molybdenum trioxide solution and belongs to the technical field of preparation of chemical products. According to the preparation method, a solution with certain concentration is prepared from molybdenyl acetylacetonate and isopropanol which are taken as raw materials, the solution is placed in a water bath kettle and heated for reaction for hours, and then the stable molybdenum trioxide solution with uniform fibrousness is obtained through extraction and sealed storage. The preparation method of the molybdenum trioxide solution is simple and convenient to operate, easy to control, and the obtained molybdenum trioxide solution is nanoscale, has excellent visible light absorbability and is relatively stable.