The plastic softening of internal friction angle exerts a certain influence on the plastic region distribution of coal surrounding borehole, and accurate characterization of the plastic region is of great significance for evaluating the stability and safety of deep coal. Firstly, based on the Mohr-Coulomb criterion and 20 groups of publicly published triaxial compression test data, the peak strength and residual strength parameters were extracted. It shows that the internal friction angle exhibits evolutionary trends of decrease, increase, and stability during the softening stage. Secondly, the Drucker-Prager strength theory was introduced to establish a deformation calculation model for coal surrounding borehole that simultaneously considers the effects of internal friction angle softening, dilatancy, and intermediate principal stress. Finally, numerical simulations and field measurements were conducted to verify the influence of internal friction angle softening on the plastic region distribution. The results indicate that: When the lateral pressure coefficient λ>1, the influence of internal friction angle softening on the deformation of the upper and lower edges of the borehole is greater than that on both sides, and vice versa; The polar angle θ=30° is the dividing point for the growth rate of the plastic region. Beyond this dividing point, the plastic radius shows a nonlinear increase, and the smaller the internal friction angle, the greater the growth rate; Compared with the plastic region obtained by only considering cohesion softening, the plastic region considering both internal friction angle and cohesion softening is closer to the field measured results, which confirms the importance of internal friction angle softening. The relevant conclusions provide useful references for the stress monitoring and safety assessment of coal surrounding borehole.