With the rapid development of unmanned aerial vehicle（UAV）， UAVs have been widely used in civil and military fields. Small UAVs with “low flight height， slow flight speed and small dimensions” characteristics are especially convenient and flexible to use and relatively inexpensive. They can be used for reconnaissance， surveillance，suicide attack and other task purposes， posing a serious threat to daily air defense security and important assets protection. The main threat patterns of the small reconnaissance UAV and payload capabilities are analized. The denial distance requirements to small UAVs are also proposed， which can provide reference for anti-UAV system development .