For decreasing the false alert in high flight density, we researched the position-velocity vector for aircraft pairs, and proved that only the pairs which inner product of difference vectors of position and velocity was less than zero would be in unsafe state. According to the flight rules, the pairs were be confirmed by linear prediction. Many scenarios via simulation surveillance radar data were demonstrated and the results showed that it was more useful and reliable for high flight density situation, and especially in reducing the false alert for opposite flying aircraft
