Circulation system of liquid laser medium provides an effective method of eliminating thermal effect and improving laser beam quality. Based on turbulent flow field characteristic of this laser medium and numerical simulation using software Fluent6.53, the relationship between flow velocity, turbulent intensity, boundary layer thickness and temperature gradient has been investigated, which affects on thermally induced distortion. Increasing velocity of laser medium in the cell would reduce thermally induced distortion and boundary layer thickness. Setting proper aperture under current conditions can also further reduce the distortion. The maximum optical path difference decreases from 10 wavelengths to 5 wavelengths at the flow velocity of 20 m/s by setting aperture.