A terahertz quantum cascade laser (THz-QCL) is a kind of semiconductor laser, which is suitable to realize the self-mixing interference. In this work, an experimental method based on self-mixing interference effect is demonstrated for the measurements of the emission spectrum, linewidth enhancement factor, and feedback optical coupling coefficient of a THz-QCL. We set up the optical path for self-mixing interferometry with THz-QCL to obtain the self-mixing interference signal with high signal-to-noise ratio as well as its variance with feedback optical distance, based on voltage driven by THz-QCL. By analyzing the self-mixing interference signal, the lasing spectra of THz-QCL under different operating currents and temperatures are obtained accurately. The resolution of spectra is inversely proportional to feedback optical distance. In addition, we investigate the linewidth enhancement factor of THz-QCL and the feedback optical coupling coefficient based on the self-mixing interference signal. The self-mixing interferometry demonstrated here is expected to be further developed into one used for terahertz spectral identification and measurement.