The shear deformation of rock joints is significant for the safety and stability of rock engineering. In order to study the constitutive relation of shear deformation in jointed rock mass under a normal stress, the direct shear tests under different normal stresses were carried out on the sandstone specimens with irregular joints using the RDS-200 rock direct shear test system. Based on the shear stress-deformation curve of the jointed rock, it can be divided into four stages including pre-peak compaction stage, linear stage, yield stage and post-peak softening stage. The post-peak softening stage can be further divided into three types including platform type, gradual decline type and drop type based on the decreasing magnitude and rate of shear stress at the post-peak period. The shear deformation constitutive model of the sandstone with irregular joints was established using piecewise function based on the shear deformation characteristics at different stages. Compared with the existing constitutive models, the new proposed shear deformation constitutive model of the jointed rock mass has a much higher fitting accuracy for the experimental results, which can better describe the deformation characteristics of the jointed rock in the whole shear process. The shear stress-shear displacement curve of irregular joints with different roughness coefficients under different normal stresses can be predicted after determining the relevant model parameters in the corresponding empirical formula after some direct shear tests. The research is practical for understanding the shear deformation of joints in rocks by numerical simulation and the safety evaluation of engineering.