This investigation applied the digital image correlation technique (DIC) on a pressure vessel that contained several surface pit defects under high pressure. Data on the deformation of the defects and peripheral area is obtained by this method. The results show that the stress and strain increase with the depth among different pits and are the largest at the bottom of any given pit. This method has proven to be a good choice for this type of experiment, where elastic and plastic surface strains need to be measured. The DIC can satisfy the requirements of being in situ, in real time, full-field and make non-contact measurements with more accurate and obvious experimental results compared with traditional measurement methods and pressure vessel test regulations. Also, it is a new, effective way for monitoring defects in online pressure vessels as well as a reliable basis for pressure vessels’ safety evaluation.