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A theoretical model of cascaded terahertz (THz) difference-frequency generation is established based on one-dimensional coupled-wave equations. The relationships between sphalerite crystals’ wave vector mismatches and difference-frequency pump waves are analyzed. To produce terahertz wave with the frequency of 1.5 THz, 80-order cascaded difference-frequency is applied. By introducing crystal absorption, we calculate the optimum crystal length and pump frequency under actual circumstances. It is found that Stokes waves dominate the terahertz waves output in cascaded progress, and cascaded difference-frequency can increase the photon conversion efficiency obviously.