The invention discloses a method of preparing copper-cobalt-tin-sulfur nanocrystal by molten salt growth and belongs to the field of thin-film solar cells. The method comprises mixing a copper salt, atin salt, a cobalt salt and a sulfur source with molten salt to obtain a mixture; roasting the mixture in a protective atmosphere, crushing, and washing to obtain CCTS (copper cobalt tin sulfur) nanocrystal with high purity stannite structure. The method is simple, low in cost and suitable for large-scale production and has good experimental repeatability and stability; the finished nanocrystal has good crystallinity and high purity and accordingly is applicable to the preparation of a copper-cobalt-tin-sulfur absorbing layer material matching with thin-film solar cells in terms of bandgap. The finished nanocrystal has form and composition which can be controlled by adjusting reaction temperature and molten salt content; cryptal particles in the molten salt are grown in balanced state; therefore, the nanocrystal grown by means of the molten salt growth is better than absorbing layer materials prepared in traditional modes in terms of characteristics.