This study incorporates the concept of deflection curves with displacement influence lines in order to propose a new concept of quasi-static deflection surface for short-to-medium span bridges under moving loads. Analytical formulas of deflection surfaces are derived using the principle of virtual work. An alternative form by deflection matrix is given for practical applications. After that， the deflection surface difference of a simply supported beam， as well as a continuous box-girder bridge before and after damage， was used to locate damage. Lastly， the superstructure model of a continuous box girder bridge with variable cross sections was statically tested in the laboratory. Measured deflections were used to construct the deflection surfaces of the box girder bridge model before and after damage. The artificial damage was successfully located using the difference of these deflection surfaces. The analysis results demonstrate that the proposed method is also applicable to box girders with variable sections. Damage localization can be achieved using structural measured deflection data before and after damage. The peaks on a deflection difference surface correspond to the damage locations， and the peak magnitudes reflect the changes in damage severity to some extent.