We propose an ex-situ reposition error compensation method to solve the reposition error in the ex-situ state of 2D digital image correlation (2D-DIC) in the long-term deformation of the measurement specimen or under special load deformation. Using the dual-reflector auxiliary measurement system with an included angle of 90 deg, the two sides of the repositioned specimen are mapped into left and right mirrors. The in- and out-of-plane rigid body displacements are compensated via the error compensation method, and the real in-plane displacement of the tested specimen is obtained. The accuracy and practicability of this method are verified by conducting an in-plane rigid body displacement experiment, unloaded specimen reposition experiment, thermal deformation experiment, and finite element analysis. We provide a simple, effective, and high-precision deformation measurement method for 2D-DIC to realize ex-situ measurement.