Based on deflection theory (DT), the basic differential equations and deformation compatibility equations of self-anchored suspension bridges (SASBs) are established. The calculation formula for the stiffening girder (SG) deflection of a three-span SASB is derived, and the validity of the formula is confirmed through an example. In addition, a fast calculation method for the theoretical deflection influence line (TDIL) is proposed by using the deflection calculation formula, and a calculation method for the actual deflection influence line (ADIL) is founded on the concept of time and space coordinate transformation. By loading the bridge with a load car and using real-time monitoring of the displacement response of the bridge test section, combined with the proposed intelligent damage evaluation program, the abnormal situation of the damage of the three-span SASB can be quickly and accurately identified. The research results of this paper can provide flexible, economical and effective guidance for detecting the capacity of real bridges.