In order to improve auto-focus performance of self developed“cat′ s eye”detection system, three auto-focus performance influence factors are analyzed, which are speckle noise in active laser images, autofocus windows and field of view. According to the three factors, modified auto-focus algorithm and strategy are proposed. The experimental results show that the modified methods indeed improve auto-focus performance. Furthermore, it provides vivid and precise active laser images of thread targets in order for subsequent early warning of the detection system.