Abstract: The aerodynamic characteristics of an airfoil in rain were studied using computational fluid dynamics method.The flow field was calculated by solving the steady compressible Navier-Stokes equations.The trajectories of raindrops in the flow field were tracked with Lagrange method and the collection efficiency of airfoil surfaces was obtained.The model of water film was also set up,then assuming that water film accumulates layer-by-layer in the direction normal to the airfoil surface and the changes of airfoil shape due to rain and the roughness height on water film were predicted.The influence of rain on aerodynamic characteristics of the airfoil was studied using k-ω SST(shear-stress transport) two equations turbulence model with enhanced wall treatment.The results show that rain effect on the lift coefficient and drag coefficient of the airfoil is little within the low angles of attack;rain can cause boundary airflow separation on upper surface and thus result in serious aerodynamic penalties when the airfoil is nearly stalled.