The design parameters for the wing anti-icing system (WAIS) are determined with the flight conditions and atmospheric and meteorological conditions. To meet such a variety of conditions, the worst conditions are generally considered for the WAIS design. In this paper an approach is proposed to determine the design parameters for the WAIS, ie critical design points (CDP). In the framework of the Eulerian method, both of the air flow field and droplet impingement behavior are obtained by numerically solving two sets of Navier-Stokes equations. As the result, the heat flux requested for dry surface conditions is estimated, on the other hand, the heat flux supplied for icing prevention by the piccolo tube system (PTS) can be calculated based on the first law of thermodynamics. Then the distributions of connective heat transfer coefficients and droplet impinging mass flux along the wingspan are predicted.