Abstract: Supercooled large droplet (SLD) is beyond the envelope described in the Appendix C of FAR 25.When the droplets impact on the surface of airfoil, they will run downstream and freeze to shape of ridge. SLD runback ice may endanger the safety of the aircraft significantly, but the aerodynamic effect of icing on vehicle performance is not completely understand. The numerical simulation of SLD icing is carried out using Reynolds Stress Model (RSM) in this paper. The computational maximum lift coefficients and stalling angles are agree with Lee's experimental data on NACA 23012m, the applicablitiy of RSM in analysis of flow separation resulting from runback ice is valialated. The impact degree of runback ice is quite different according to airfoil geometry, and the supercritical airfoils and tail airfoils are specially analyzed in large civil aircraft. The results demonstrate that runback ice influences the flow separation and significant deterionates aerodynamic performance of supercritical airfoils. For the tail airfoils, the aerodynamic impact of SLD icing is not so severe , and the size of separation region is diminished at higher angles of attack. The contents of this paper have reference values in aircraft design and airworthiness certification for large civil aircraft.