The characteristic absorption spectra of a large number of substances are in the terahertz range, so the development of terahertz spectroscopy applications has attracted much attention in recent years. Compared to existing commercial spectrometers, spectral measurement methods based on tunable single-mode lasers offer the advantages of high accuracy and high spectral acquisition speed. Terahertz quantum cascade laser (QCL) is an ideal candidate for tunable laser sources. Its tuning characteristics need to be studied before it can be used to achieve spectral measurements, but the existing measurement methods are limited by accuracy. The study finds that the corresponding beatnote signals can be obtained in the microwave band by beating the terahertz quantum cascade optical frequency comb and the single-mode laser. When the single-mode laser is tuned, the beatnote signals shift accordingly. Therefore, combined with the self-detection technology of QCLs, the tuning coefficient of the single mode laser can be measured with high precision by measuring the microwave beatnote signals using a spectrum analyzer. The resulting coefficient of the measured terahertz single-mode laser were 53 MHz/K (temperature tuning) and 2.7 MHz/mA (current tuning).