Terahertz (THz) generation by periodically-poled KTiOPO4 (PPKTP) with a quasi-phase-matching schemebased on cascaded difference frequency generation (DFG) processes is theoretically analyzed. The cascadedStokes interaction processes and the cascaded anti-Stokes interaction processes are investigated fromcoupled wave equations. THz intensities and quantum conversion efficiency are calculated. Compared withnon-cascaded DFG processes, THz intensities from 10-order cascaded DFG processes are increased to 5.53. The quantum conversion efficiency of 479.4% in cascaded processes, which exceeds the Manley-Rowelimit, can be realized.