To improve the sub-pixel registration accuracy of quadratic surface fitting method in digital image correlation, the method was corrected and the corrected quadratic surface fitting method was used to research the effect of sizes and contents of WC on the thermal expansion coefficients of WC/Cu composites. First, the measurement error sources of the quadratic surface fitting method were analyzed in a practical application. Then, WC/Cu composites were prepared by a powder metallurgy method, and thermal expansion tests of the specimen with speckles which can resistant to high temperatures were performed and thermal deformation fields of the WC/Cu Composites under different temperatures were measured by the corrected quadratic surface fitting method. Finally, the truth values of the thermal expansion coefficients were given by quadratic polynomials fitting. The experiment result indicates that corrected method effectively improves sub-pixel registration accuracy, especially reduces the displacement fluctuation around 0.5 pixels, and offers more accurate measurement results.