Abstract: The effect of the degree of rivet opening on the rigidity of the interlock in 5052 a-alloy self-piercing riveting joints was analyzed by using image processing method. Two type joints with different rivet opening were tested in the proper test machine. The CCD with digital microscope was used to capture the images of the interlock deformation in the loading process. The relative slip between the rivet tip and the bottom sheet was calculated by using correlation analysis method, and the amount of the slip was defined as a measure of stiffness of SPR joints. Finite element models of two type joints with different rivet opening were established to simulate the deformation process of the interlock in tensile test. The results show that a critical value of slip-loading exists in the loading process. When the load value is lower than the critical value, the structural integrity of the interlock is kept and no apparent slip in the interlock can be detected. When the load value is greater than the critical value, the slip in the interlock can be observed and it increases with the increasing of the loading, and the rivet is pulled out, which is named pull-out failure model. The open degree of rivets has a large effect on the strength and rigidity of SPR structure, and high rivet degree can increase the stiffness of SPR joints.