The invention relates to a preparation method of solid vanadyl sulfate. The preparation method comprises the following steps: roasting and leaching steelmaking converter vanadium slag to obtain a vanadium-containing solution; carrying out targeted iron removal and vanadium valence conversion, carrying out impurity removal and purification by using a solvent extraction method, and carrying out mixed acid back extraction to obtain a high-purity vanadyl sulfate solution; and finally, carrying out direct negative pressure distillation crystallization to obtain the solid vanadyl sulfate. The methodis simple in technological process and low in cost, and large-scale industrial preparation can be achieved. A tetravalent vanadium solution is purified by adopting a solvent extraction method for impurity removal and purification, and impurity elements such as iron, manganese, chromium, zinc, calcium and magnesium can be effectively removed by controlling proper technical parameter conditions.