In order to resolve the safety requirement of unmanned aerial vehicle (UAV) sense and avoid system in the air traffic, some nested zones around the UAV according to its sensing capacity in the air traffic are defined. By using the differential game and the viability theory in this problem, the UAV sense and avoid problem based on these theory is defined and analyzed, and a theory framework and a solution scheme are proposed on the basis of the above work. As a conclusion, a flight strategy and guidance law associated with the safety zones is given. The uncertainty margin on other traffic intruder trajectories is reduced, by adopting some effective evasion maneuvers if only necessary separation with other aircrafts is held. Consequently, the flight trajectory is adjusted to avoid a flight conflict.