Abstract: This paper presented the aerodynamic design process for a low pressure transonic fan stage.The solver based on three-dimensional(3-D) viscous Navier-Stokes equations has been adopted as a main design tool with coarse and thick mesh,although the initial blade was sourced from the solution of S2 inverse solver.This design method eliminated the reliance on loss models in quasi 3-D design for the designer.Based on the design result,the flow field structure in the fan stage was also studied in this paper.The main flow loss of the rotor was originated from the shock and the interaction among shock,leakage flow and boundary layer.The 3-D stator design has achieved successfully the high subsonic airflow with high turning angle.