In any off-axis holographic experiments, it is generally difficult to accurately obtain plane reference wave angle so that tilt phase aberration (TPA) occurs in three-dimensional phase reconstruction for the object. In this paper, a novel approach to accurately determining the plane reference wave angle for phase reconstruction of the object in digital offaxis holography is described. The method ingeniously constructs a numerical reference plane (NRP) reflecting true tilt of the reconstructed object by randomly choosing three points from a local flat of the reconstructed object image, and establishes the relation between NRP tilt and plane reference wave angle. So the reference wave angle can be exactly obtained by iterative computation and TPA is completely compensated. The experimental result approves of theoretical prediction very well.