In order to improve the warship formations effectiveness on air defense early warning the ellipse forming principle was introduced and the ellipse plane of the air attack course was determined based on analysis to the counter-situation factors between friend and foe. Then a method for assessing the potential air threat to the warship formation was proposed by use of the analytic geometry approach. According to the information about the disposition of air assault forces the potential air assault direction sector and the interval of time can be assessed by the proposed method before air threats are formed. The application example and the simulation show the feasibility accuracy and validity of the methods.