Deflection is an important index for determining bridge stability and safety after an earthquake, but the measurement of bridge deflection is complex and suffers a lot of interference, so the obtained results are problematic. In this study, we designed a method for measuring bridge deflection after an earthquake. Based on the relevant parameters of an actual bridge, we used the conjugate beam method to calculate the initial deflection of a fixed point on a bridge after earthquake. Combined with the deflection curve function of the post-earthquake bridge, we can determine the deflection of the bridge section. To determine the deflection error of a post-earthquake bridge and correct the measured bridge deflection, we replaced the deflection error problem with the difference between the elevations of the monitored and reference points. Experiments involving simply supported and continuous girders verified the accuracy of the proposed method for measuring the deflection of a bridge post-earthquake.