This work investigates transmission characteristics of flexible solid circular polymer dielectric waveguides (SCPDWs) in the sub-THz band, especially those important for practical applications, including transmission loss, energy confinement and bending loss. In addition, the expanded polyethylene (EPE) cladding and interconnector are introduced to ensure good working conditions and quickly extend transmission length, respectively. The 1.5-mm-diameter SCPDW made of properly selected polytetrafluoroethylene (PTFE) is considered, which is flexible and low-loss. The simulation shows that more than 99% power can be confined in the 9-mm-diameter circular area when the frequency is over 90 GHz. The prototypes are fabricated and measured within 88-140 GHz. The measurement shows that the PTFE SCPDW has 0.5-4.8 dB/m loss, much lower than most reported solid polymer dielectric waveguides (SPDWs). A 30-mm-radius 90° bending is generally lossless in frequencies over 120 GHz, and adopting the polarization orthogonal to the bending direction can improve the bending loss. A 9-mm-diameter EPE cladding brings about 1 dB/m attenuation, which, however, dramatically reduces external interference on the PTFE SCPDW. The designed interconnector can easily extend the SCPDW transmission length with an acceptable ≤2 dB loss. The above results prove that the PTFE SCPDW and its corresponding cladding and interconnector can achieve good sub-THz high-speed wired interconnects. Though only a specific diameter is considered, the performances of SCPDWs with different diameters can also be deduced. The investigated features are expected to be beneficial for future SPDWs applications.