Abstract: Collinear interaction of pump wave, Stokes wave and terahertz wave was proposed in this letter to solve the problem of intense absorption of terahertz wave in gain medium in terahertz parametric oscillation. The amplification characteristics of forward and backward terahertz wave were theoretically analyzed by solving coupled wave equations under difference approximate conditions based on PPLN crystal as an example. The results indicate that the terahertz wave power can be greatly enhanced by the collinear interaction of the pump wave, the Stokes wave and the terahertz wave as the absorption coefficient are greatly more than the gain coefficient of the terahertz wave. The analysis results in this letter provide comprehensive theoretical foundations for the experiment researches of terahertz wave parametric oscillation in PPLN crystal.