A novel method for 3-D displacement measurement is presented in the paper by using isolating out-of-plane displacement from in-plane displacement. A reference is added to the dual-beam symmetrical illumination system and shared by the two illumination beams, which are separately used to illuminate the test object. With the help of phase-shifting techniques, two phase maps that include out-of-plane and in-plane displacement can be obtained. Theoretical analysis shows that out-of-plane displacement can be separated from in-plane displacement only by simple calculation of phase. In perpendicular the in-plane displacement can be measured directly by dual-beam symmetrical illumination system. When the technique is applied to an oil pump in diesel engine, 3-D displacement fields of the pump are successfully measured. It shows that this technique provides a new effective approach to 3-D displacement measurement and a new method for intensity and rigid of diesel engine elements analysis.