The 2D/3D Finite Element(FE) heat dissipation model is established for calculating the temperature distribution and heat flux of GaAs/AlGaAs Terahertz Quantum Cascade Lasers(THz QCLs) in order to investigate the thermal transfer process and effective heat dissipation methods. The influences of device geometry, heat sink materials and AlN film on thermal properties of THz QCLs are studied. The results show that the horizontal temperature distribution of active region is uniform, while the perpendicular temperature difference is high. The heat generated in active region is mainly transmitted by the heat sink. Thinning the active region, increasing the cavity length and narrowing the ridge width facilitate the heat dissipation and decrease the temperature of the active region. Adding AlN films on top of the device shows a significant auxiliary heat dissipation effect. Especially, when the thickness of the film is greater than 8 μm, the drop in temperature of the active region tends to be slow.