A fast method for detecting the frequency stability of Back Wave Oscillators (BWOs) is proposed. The radiation from a BWO is collected and collimated by a parabolic mirror and is detected by a fast terahertz detector at the other end of the collimated light beam. The measured signal is displayed and recorded via an oscilloscope. Finally, the single-frequency output stability of the BWO is estimated according to the Fabry-Perot interference principle. The method proposed is of important application value to the performance improvement of frequency-domain terahertz spectroscopy systems with high frequency resolution.