This study presented the details of a specially designed biaxial testing system with a flexible boundary to examine the features of particle motion and associated contact movement during shearing. The whole testing system was modified from a CKC triaxial testing system so that the production cost was low, and this approach was applicable to other types of triaxial testing systems. The 3D printing technique was applied to ease manufacture of the testing device and to improve the resolution of the subsequent image analyses. The middle part of the biaxial cell was printed using the 3D printer because it is often difficult to make by the traditional means due to a complicated geometry and the requirement for a one-piece material to ensure the cell is seamless and leak-proof. A packing of elliptical rods, also produced by the 3D printer, is used as the test sample. In principle, any shape of particle with designed properties can be printed. The goal of tracing the particle motion and movement at contacts during shearing is achieved by means of particle image velocimetry (PIV) and close-range photogrammetry, based on the reference dots marked on the observation windows of the biaxial cell and two object dots printed on each rod. The high-resolution 3D printer ensured the accuracy of marking the object dots on the rod and therefore improved the resolution of subsequent analyses. The experimental results demonstrated the validity of the biaxial testing system and the ability and effectiveness of the system to capture the kinematic features of particles and associated contact movements in response to biaxial shearing.