In order to study the hydraulic characteristics of unilaterally divergent stilling basins under different diffusion angles, a numerical simulation was carried out using the TruVOF method and the RNG k-ε turbulence model based on FLOW-3D. The characteristics of the velocity and vortex was investigated as the focus of the study, and the calculation results of the surface line, time-averaged pressure were compared with those of the physical model test. The comparison results show that the numerical simulation results are basically consistent with those obtained by model test. On this basis, an in-depth study found that the velocity on the bottom of the stilling basin is unevenly distributed, with a larger velocity on the non-diffusion side. With the increase of the diffusion angle, the maximum near-bottom velocity gradually moved to the non-diffusion side. A vertical vortex formed on the diffusion side and gradually reached the slab of the stilling basin with the increase of the diffusion angle, and the scale of the vortex kept increasing with a stable position. The strong vortex segment appeared in the middle of the vortex and presented the regularities of distribution of small at both ends and large in the middle along the axial direction.