In order to study the law of incompressible radial diffusion flow between two parallel circular plates,starting from the momentum integral equation,a new formula for the variation of dimensionless boundary layer thickness and dimensionless radius ratio between parallel gap in a disk thrust gas bearing with double symmetrical contraction section was presented.An approximate solution of the velocity distribution of the radial flow between two parallel plates was obtained by using the power velocity hypothesis in the turbulent boundary layer.The velocity distribution in parallel gap of high pressure disk gas bearing was calculated by numerical simulation,and the simulation result was compared with the approximate solution.The result shows that the new formula can accurately reflect the development process of the boundary layer thickness,so that the velocity distribution characteristics in parallel gap can be quickly obtained.