Abstract: Structural crack change is one of the important bases to measure the safety state of concrete dams. In view of the problem that the measurement accuracy of traditional vision measurement methods is significantly reduced under the condition of small angle tilt shooting, based on the equivalent physical model of structural joint change, a new target which can adapt to small angle shooting is designed, and a two-dimensional plane change detection algorithm of structural joint is proposed in this paper. This method uses the cross ratio invariance and the property of vanishing point in projective geometry to correct the perspective deviation of the concentric center of the target, so as to obtain the accurate coordinates of the target feature points, and deduce the change detection algorithm of the structural seam in the x and y directions. In this study, the target is placed on the slide to simulate the changes of structural joints, and the method is tested from many aspects. The indoor verification test results show that the measurement errors of this method in x and y directions are within 0.1 mm under different shooting angles, even small angle shooting conditions. In the field test, the errors in x and y directions are within 0.25 mm, which meets the accuracy requirements of the specification. It can realize the accurate detection of two-dimensional plane changes of structural joints under the condition of small angle shooting, and has engineering application value.