Surface flatness is an essential indicator for quality assessment of concrete surfaces in the construction industry. Terrestrial laser scanning (TLS) has been popularly applied for surface flatness inspection due to its speed and accuracy. However, scanning area far away from the TLS usually suffers from inaccurate surface flatness measurement and physical barriers such as interior walls are likely to cause occlusion for the surface flatness inspection. To address these limitations, this study proposes a mirror-aided technique for surface flatness inspection. There are two concepts proposed from the mirror-aided technique. First, the mirror-aided approach can address the low accuracy of surface flatness inspection caused by long scanning range and high incident angle, which enlarge the scanning area and increase the surface flatness inspection efficiency. Second, the mirror-aided approach can measure the flatness of the concrete surfaces occluded by barriers based on the mirror reflection principle with one single scan, resulting in efficient surface flatness inspection. To validate the proposed two concepts, a series of tests on laboratory-scale specimens are conducted. The results show that the proposed method provide accurate surface flatness estimation results with an accuracy of more than 85% for the scanning area far away from the TLS and concrete surfaces with occlusion problem, demonstrating the great potential for the application of mirror-aided surface flatness inspection in the construction industry.