A runaway speed of servo system is an ordinary fault. It will strike and damage the mechanical limit in an axis system with limited rotation. The optical machine structure and the cable would be destroyed. Currently, a traditional software spacing is used to restrain runaway speed in servo control system, but it is limited. Software spacing will be invalid when control software crash. In order to solve the runaway problem, the causes and the principles of runaway speed of servo system are analysed in this paper. Several new methods to avoid runaway speed are presented. Especially, the bottom hardware circuit design to avoid runaway speed is effective and reliable. It is an effective method to avoid runaway speed of servo system and has been validated in the application of optical-electronic equipments.