<p>Understanding the dynamic shear characteristics of infilled joints is crucial for the design, construction, and maintenance of geotechnical engineering projects. However, the dynamic shear behavior of infilled joints under impact load remains insufficiently explored. In this study, a series of impact-induced direct shear tests have been carried out on infilled planar granite joints and clean joints employing the biaxial Hopkinson pressure bar system. The effect of normal stress and shear rate on dynamic shear characteristics of infilled joints was investigated. The grain comminution behavior of filling during the test was analyzed. Additionally, the practical significance, limitations, and prospects of this study were discussed. The result shows that the dynamic shear stress–shear displacement curve of infilled joints can be divided into compaction phase, shear stress accumulation phase, slip phase, and rebound phase. The dynamic shear strength and dynamic friction coefficient of the infilled joints and the clean joints both increase with increasing normal stress. With the increase of shear rate, the dynamic shear strength of the infilled jointed specimens and the clean jointed specimens both decrease. Infilled joints are typically characterized by low shear strength and high deformability; specifically, the dynamic friction coefficient of infilled joints is reduced by 46% compared to clean joints, while the maximum shear displacement is increased by 168%. Grain comminution plays a pivotal role in determining the shear mechanical behavior of infilled joints.</p>