The suction stress profile is of obvious nonlinear characteristics under steady flow condition. This study supplemented the Prandtl assumption under the condition of considering the single vertical seepage and deduced the calculation model of bearing capacity for foundation of unsaturated soil under steady flow condition by means of rigid body balance method in the range of Prandtl sliding surface. It also discussed the influence of change of the groundwater and ratio flow on bearing capacity. The results show that the bearing capacity formula of unsaturated soil foundation under steady flow can take into account the influence of the nonlinear distribution of the soil suction stress; when suction stress increases firstly, and then decreases with the depth, the bearing capacity decreases first, and then increases as groundwater lever is lowered.