The cracking of concrete plays a vital important role for the serviceability and durability of reinforced concrete (RC) structures in terms of the structural behavior. To investigate the crack behavior of concrete structure with additional strengthening, monotonic four-point bending test was carried out for RC beams which strengthened with near surface mounted (NSM) prestressing screw-thread steel bars (PSB). Detailed description of the strengthened beams was presented, particularly on the crack initiation and propagation, the distribution, even the crack width and length, based on the deformation fields obtained from the non-contact digital image correlation (DIC) method. In addition, the relationship between the crack width and the damage level was discussed, and based on the results a crack-based damage assessment method for NSM strengthened members was proposed. The results reveal that the beam strengthened with NSM PSB steel bars demonstrated higher load carrying capacity than the reference beam. The prestressing application could significantly increase the cracking load thus delaying the initiation of cracks. The beams NSM strengthened with prestressed bars exhibited more flexural cracks in the constant moment region with a smaller crack width and spacing compared to the specimen without prestress. Furthermore, the crack-based damage assessment method proposed in this paper was demonstrated to be feasible to NSM strengthened beam for determining the damage status and repairability with simple operation.