Abstract: The proposal of the free flight has brought tremendous pressure on air traffic safety and put forward higher requirements for conflict detection technology. The traditional speed obstacle model is dedicated to detecting and resolving potential flight conflicts on air routes. Therefore, this paper proposes a three-dimensional deterministic conflict detection algorithm based on the speed obstacle method. On the basis of the ellipsoidal protection zone, the current position, speed vector, and heading of the aircraft are selected as the characteristic quantities, and the geometric solution of the two aircraft is solved based on the speed obstacle method. The conflict status is qualitatively judged, and the conflict detection model is established. The simulation results show that this algorithm can effectively detect potential flight conflicts between two aircraft in three-dimensional space without considering other conditions.