Abstract: Critical ice shape determination method and its effects on aerodynamics were investigated. A typical tool used in the development and certification of an aircraft for flight-into-known-icing conditions is critical ice shapes. These shapes are developed for both the unprotected and protected surfaces of the aircraft. They are then used in dry-air testing to assess the likely worst case handling and performance of the aircraft in natural icing flight. Critical ice shapes are those with ice accretion geometries and features representative of that which can be produced within the icing certification envelope that result in the largest adverse effects on performance and handling qualities over the applicable phases of flight of the aircraft. A critical ice shape determination method was developed based on CFD approach, which was applied to critical ice shape condition analysis, sensitivity section determination, icing parameter sensitivity analysis, critical icing condition determination and critical ice shape determination. The flow field was calculated using CFD platform of AVIC Aerodynamics Research Institute (UNSMB), by solving N-S equation based on the finite volume method with Jameson central scheme. Impingement characteristic of droplets was calculated using the Eulerian method for the droplet trajectory equation, and classical Messinger thermodynamics model was applied to simulate ice growth. Sensitivity of ice shapes to parameters as ambient temperature, droplet diameter, and flight conditions were analyzed on Common Research Model (CRM) model, with the 50% spanwise wing section as the sensitivity analysis section. An airfoil sensitivity approach was applied to determine critical icing condition and ice shape for CRM, by comparing the ice geometry parameters such as ice horn angle and ice thickness. The 30 micron case has the largest and most after ice thickness and ice horn angle, the maximum ice thickness is about 41mm. Regarding aerodynamic degradation due to calculated ice, critical ice shape affects seriously the aerodynamic performance of aircraft, causes that lift reduces by 17%~70.9%, drag increases by 6.7%~23.8%. The developed 45 minutes holding critical ice shape determination method can be used to numerically determine critical condition and ice shape, and is applicable in civil aircraft design and certification.