In order to detect the phase error caused by platform vibration of synthetic aperture lidar, a new method of vibration measurement was proposed, which extracted the detection signal phase directly with heterodyne balanced detection and quadrature signal processing technology. Then the errors caused by the unbalance between the two quadrature signals were corrected and the obtained phase sensitivity was 1.45 rad. The experiment showed that this method not only can accurately measure phase variation caused by small vibration with low frequency, but also was insensitive to the amplitude of echo signals′ fluctuation, which was different from the laser Doppler vibrometer.