A method for conflict detection of collision avoidance and assessment of collision threat levels are presented in this paper.The principle of collision avoidance was introduced firstly.In order to obtain an effective result different strategies for collision avoidance were established based respectively on heading control velocity control height control and the combination of heading and velocity.The models of multi-objective cost function and the methods of optimal decision-making for automatic collision avoidance were also established.The simulation results demonstrated the effectiveness and practicability of the proposed methods.