Power-aware X-filling is a preferable approach to avoiding IR-drop-induced yield loss in at-speed scan testing without the need of any circuit modification. However, the effect achieved by previous X-filling methods for reducing launch switching activity may be far from optimal. In addition, some of them are not scalable. This paper proposes a new X-filling method based on the genetic algorithm (GA). Experimental results on benchmark circuits demonstrate the effectiveness and scalability of the new X-filling method for reducing launch switching activity.