The invention discloses a terahertz wave parametric oscillator based on the cascading parameter effect. After pump light emitted from a pumping source is subjected to bean shrink through a telescope beam shrink system, the pump light enters a MgO:LiNbO3 crystal after passing a reflector and a light beam scanner and is totally reflected in the MgO:LiNbO3 crystal; the pump light generates first-order Stokes light and terahertz waves in the MgO:LiNbO3 crystal through the optical parameter effect, the wave vector of the terahertz waves is perpendicular to the exit face of the MgO:LiNbO3 crystal, and the terahertz waves directly exit from the MgO:LiNbO3 crystal; the cascading parameter process can occur all the time, all orders of Stokes light have respective resonant cavities, the specific area of the side face of the MgO:LiNbO3 crystal is plated with an all-order Stokes light total reflecting film, and the terahertz waves are continuously amplified in the cascading parameter process. The wave vector of the pump light and the wave vector of all orders of Stokes light are both perpendicular to the exit face of the MgO:LiNbO3 crystal, the losses of the pump light and all orders of Stokes light are effectively reduced, the incident angle of the pump light is changed by changing the direction of the light beam scanner, therefore, the included angle between the pump light and the axis of the first-order stokes light resonant cavity is changed, frequency tuning output of the terahertz waves can be achieved, and operation is flexible.