In order to improve the operating efficiency of closely spaced parallel runways, safe zones of paired approaches is analyzed. Combined with the relative motion relationship and speed characteristics of paired aircraft, paired approaches is divided into fast aircraft can not pass the slow aircraft pairing mode and fast aircraft beyond the slow aircraft pairing mode. Regarding the slow aircraft as the reference volume, the relative motion state of the fast is analyzed, and the safe area of the two pairing modes is given qualitatively. And then, taking account blunder, the model of collision-safety boundary is established. Meanwhile, considering the effect of the lead aircraft to the trailing because of side wind, the model of collisionsafety boundary is established. The model is verified by using the C class aircraft B737-800 and the D class of aircraft B747-400 as the paired aircraft on the base of relevant data of Hongqiao international airport. The results show that the proposed method can be used to quantitatively calculate the safe zones of the paired approaches in real time.