Self-mixing interference in broadband terahertz(THz) Quantum Cascade Lasers(QCLs) are studied by the transition matrix theory and rate equations method. Under weak optical feedback strength, with the target moving in uniform motion, the self-mixing signal changes in a sine-like waveform. It is found that the self-mixing signal of the broadband THz QCLs under weak feedback can be applied to range finding, imaging, and spectral measurement. Under strong optical feedback, the spectrum is affected by the optical feedback obviously, while new modes arise near the solitary modes. As a target moves in uniform motion, the self-mixing signal shows the same number of peaks as that in weak feedback case. Therefore, the broadband QCLs under strong feedback cannot be applied to spectral measurement, but it can be applied to realize micron-magnitude range finding and imaging under certain conditions. The study is helpful to the application of THz sensor technology based on self-mixing interference in broadband THz QCLs.