Abstract: An unmanned aerial vehicle (UAV) launch missile from send a dictate to missile depart from orbit, a phenomenon appears: the missile persists on the orbit for too long time. This paper analyses the cause is abnormal increase of unlock force. Because the time of the missile thrust increases to unlock force is lengthened. So the missile persists on orbit is too long time. This affects the cannon-shot of the missile. The more severity instance induces launching failure indeed. It issues one amendatory mathematics model of unlock force and designs a test scheme to show the fault: the changed friction coefficient. In the end, it educes the pertinency improving measure and acquires the ideal result. It brings us to a new altitude on understanding the detent unlock force.