Based on the principle of quadrilateral element digital image correlation (Q-DIC) method, this paper proposes one element Q-DIC method, which takes the region used to correlation calculation as one element of the finite element method (FEM). Simulated images are used to verify the accuracy of this method. Since the pixel points with the maximum error are located at the node or on the boundary of the element, measured deformation of the core area (area surrounding the center point of the calculation region with a smaller size) is taken as the reliable measured value. Comparison of one element Q-DIC with various displacement hypotheses shows that the key to improve the measurement accuracy which is choosing the suitable displacement hypothesis. The region of interest (ROI) is divided into several core areas. Then, the full-field deformation of ROI could be obtained by interpolating the measured deformation of the core area. One element Q-DIC is used to measure the elastic constants of polycarbonate. And a three-point bending test is used to verify the performance of one element Q-DIC. The measured deformation is in accordance with the numerical simulation. The method proposed in this paper has a widely prospect for the practical application.