Cascaded optical parametric oscillations generating a tunable terahertz (THz) wave are analyzed to solve the problem of low quantum conversion efficiency in a THz-wave parametric oscillator. The forward and backward optical parametric processes are theoretically analyzed based on periodically poled lithium niobate (PPLN) as an example. Tuning characteristics of the THz wave that relate to the parameters of the pump wavelength, the grating period of PPLN, and the working temperature are numerically simulated. The gain and absorption characteristics of the generating THz wave are deduced in the situation of quasiphase-matching configuration at different working temperatures.