On the Basis of the long-baseline laser interferometer, an online stability monitoring technique is studied for nanoscale displacement and vibration measurement. A laser interferometer with 22 m arm is built to measure displacement and vibration. Based on phase generated carrier (PGC) demodulation technique, an indication signal with known frequency and fixed amplitude is added. Working condition monitoring of laser interferometer, measurement of environmental changes and fluctuation indication can be realized by the variation of the amplitude of indication signal. Experimental results show that online condition monitoring of the laser interferometer can be achieved by adding a known indication signal. This method can carry out online stability monitoring of laser interferometer without extra hardware load, which can simplify the debugging process of interferometer and improve the running efficiency.