Microscopic structure and diffusion properties of benzene in ambient water (298 K, 0.1 MPa) and supercritical water (673-773 K, 25-35 MPa) are investigated by molecular dynamics simulation with site-site models. it is found that at the ambient condition, the water molecules surrounding a benzene molecule form a hydrogen bond network. The hydrogen bond interaction between supercritical water molecules decreases dramatically under supercritical conditions. The diffusion coefficients of both the solute molecule and solvent molecule at supercritical conditions increase by 30-180 times than those at the ambient condition. With the temperature approaching the critical temperature, the change of diffusion coefficient with pressure becomes pronounced.