Direct shear is one of the most common laboratory testing methods to study the shear behaviour of infilled rock joints. One of the major concerns is the boundary conditions of infill material in the shear direction, or in other words the “lateral confinement” conditions of the infill layer. In this paper, the effects of various infill lateral confinement conditions on the joint shear behaviour were investigated in the software FLAC. It was found that the interface slip patterns, shear band distribution, and the variations of shear stress and lateral confining stresses, are all dependent on the selected lateral conditions. To ensure a stable model and prevent infill squeezing, two elastic blocks can be glued on the lateral edges of the infill layer. Otherwise, the lateral stress confinement (i.e. constant stress, stress equal to average infill horizontal (XX) stress, and the constant lateral stiffness conditions) are suitable for realistic modelling. Furthermore, the Constant Lateral Stiffness (CLS) conditions lead to asymmetrical confining stresses during shear, and an increase in the stiffness raises the infilled-joint shear strength exponentially.