Deflection is the basic data of bridge health monitoring. It can provide quantitative information for both structural safety evaluations and maintenance purposes. By analyzing the relationship between the displacement and its first and second derivatives (inclination and curvature), a method to detect the local stiffness damage of the structure by using the area difference of curvature of the deflection curve before and after the damage was proposed. The area enclosed by the curvature curves before and after damage was divided into several elements, and the ratio of the square of each element area to the sum of the square of all elements area was used as the damage location parameter ΔAκ. The peak value of ΔAκ can locate the damage. As a theoretical example, a simply supported T–beam bridge model with different degrees (5%～50%) and different numbers of local stiffness damage was simulated. By calculating ΔAκ value of each element, the damage in the structure was accurately detected. The method was also applied on a PMMA model bridge in the laboratory. The results showed that ΔAκ value at the damage location was much larger than that at the undamaged location, which could accurately locate the minor local stiffness damage in the structure, and ΔAκ was independent of the damage degree. The damage could be clearly located even in the case of measurement noise. This method is helpful to improve the efficiency of daily safety inspection of bridge, quantify the inspection data, objectify the identification results, and promote the application in actual bridge engineering.