Terahertz(THz) wave generation by organic crystal 2-[3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile(OH1) with a collinear phase-matching scheme based on cascaded difference frequency generation(DFG) processes was theoretically analyzed. The cascaded Stokes interaction processes and the cascaded anti-Stokes interaction processes were investigated from coupled wave equations. THz intensities and quantum conversion efficiency were calculated. Compared with non-cascaded DFG processes, THz intensities from 13-order cascaded DFG processes were increased to 15.96. The quantum conversion efficiency of 1 377% in cascaded processes can be realized, which exceeds the Manley-Rowe limit.