Firing of the OTS missile is conditional, and only by ensuring safety can a fighter use the OTS missile to attack the rear target. Therefore, while attacking the rear target the pilot of the fighter should also consider the target's threats to the fighter. Firstly, this paper analyzes several factors those threaten to fighter's safety by a threat-factor method to determine the proportion of each factor, and provides evidences for the pilot to make correct choices. The paper also briefly describes the path tangent avoidance method and illustrates the situations for adopting the method. And then the paper presents a launching opportunity of over the shoulder missile. The basic principle is to ensure that the fighter is beyond the target's threat while the target still can be hit by the OTS missile. The simulation results show that it is necessary for a pilot to make decisions under the enemy's threat, and simultaneously commanding the opportunity of launching over-the-shoulder missile is of great importance to winning the air combat.