The invention discloses a preparation method of a third-order nonlinear optical Mo/Cu/S cage type cluster-based crystalline material. The preparation method comprises the steps: adding ammonium tetrathiomolybdate and cuprous iodide into N-methyl-2-pyrrolidone to be stirred to prepare a solution; adding a buffer layer on an upper layer of the solution, and then adding a solution prepared from potassium iodide and dibenzo-18-crown-6 above the buffer layer; sealing and then crystallizing, filtering, washing and drying to obtain the crystalline material. The preparation method disclosed by the invention has the advantage that the cage type cluster-based crystalline material with third-order nonlinear optical performance is obtained. A formula of the cage type cluster-based crystalline materialis shown in the description.