The supercooled large drop regulations would increase the level of safety during flight in icing and have a certain impact on ice detection system design. Appendix O Supercooled large drop icing conditions is summarized, the brief analysis of appendix O part I-meteorology is performed, the effect of FAR25.1420 supercooled large drop icing conditions on ice detection system design is analyzed, the ice detection methods complying with related requirements are provided in this paper. If certification for flight in icing conditions is sought now, it is appropriate for an airplane with a maximum takeoff weight less than 60, 000 pounds or with reversible flight controls to certificate in accordance with FAR25.1420(a)(1). In addition, a proposal of ice detection system complying with FAR25.1420 is presented.