Experiments for measuring high temperature full-field strain were conducted during GTAW welding process using digital image correlation (DIC) technology. A special visual sensor system was equipped with image acquisition module. In order to reduce the influence of intense arc interference, a special laser-based illumination and filtering system was utilized. The in situ total strain close to the fusion line was measured during arc welding process. According to the total strain measured by DIC method and thermal strain calculated by the coefficient of thermal expansion, the mechanical strain was obtained, illustrating the sum of elastic and plastic strain distribution and deformation in the heated affected zone of the joint. Results showed that there remained compressive strain in both horizontal and vertical direction near the welding joint after welding. It demonstrated that full-field strain measurement could be achieved by DIC method during in situ arc welding.