To achieve the measurement for complicated three-dimensional object surface outline and the longitudinal displacement or vibration of object within a small space, it′s necessary to develop optical detection system with high resolution and non-contact. Laser Doppler technology is preferentially applied in longitudinal displacement measurement of complex three-dimensional object because it has high accuracy, quick dynamic response, good linearity, great measuring range and is noncontact. However, the signal got from the scattered light of object measured is faint, so improving the intensity and signal-to-noise ratio of the signal are pivotal. Laser Doppler technique and statistic law of random changing of scattered light phase are studied and a reference optics with high spatial resolution is designed. The measurement result is satisfying when the system is applied to the off-surface displacement measurement, with a relative error of 0.3%.