Cascaded optical parametric processes in terahertz parametric oscillator with a noncollinear phase-matching scheme based on bulk lithium niobate were observed. The spectra of the first-, second-, and third-order Stokes waves were observed. By analyze the spectra of the first-, second-, and third-order Stokes waves it is found that the frequency difference between adjacent-order Stokes waves is approximately equivalent, which means that the high-order cascaded parametric processes have realized. With high-order cascaded terahertz parametric processes one pump photon can generate several THz photons, which means that the quantum conversion efficiency can be enhanced.