We have investigated on a tunable graphene-based hybrid plasmonic terahertz waveguide with ultra-long-ranged propagation length at the frequency of 3 THz by using a finite-element method. The proposed structure consists of five graphene sheets, a curved semiconductor layer as a low-index material at the top of an air gap spacer, and gallium arsenide as a high-index material. The most important characteristic of this hybrid structure is its tunable, long-ranged propagation length. By tuning the waveguide dimensions, we can have a very long propagation length ranging from 20 to 15,000 μm, which is approximately 10 times higher than that in recently proposed structures. Moreover, we will show that with this increase in the propagation length, the normalized mode area does not change significantly and remains between 0.03% and 0.04%, indicating that this structure also poses a tight energy confinement.