An on-line 3D detection system for optic component surface measurement was designed based on digital holography. An off-axis structure was chosen to evade the optical axis of a test object and the tilt phase compensation was used to eliminate the tilted phase distortion caused by an off-axis optic path. In this system, the illuminating angle and view direction could be changed by rotating the object around the optical axis, by which the recording distance was increased and the resolution was improved using synthetic aperture technique. Meanwhile, the superposition of reconstruction images could decrease the effect of speckle noise on the reconstructed distribution. The proposed method is experimentally validated by a resolution test target and an optic mirror, and it shows that the 2D resolution of the system has been 10 μm at a recording distance of 40 cm and the system noise also be suppressed effectively.