We performed a systematic investigation on the dynamic behavior of conduction filaments (CFs) in WO3-x-based devices. It was found that the electric forming produced an electric structure consisted of a conductive channel (virtual cathode) started from cathode and an insulating band surrounding anode. Both the virtual cathode and the insulating region varied with repeated resistance switching. Set/reset operation affected device resistance mainly by modifying the CF, which formed in the setting process together with an insulating halo that separated it from the virtual cathode. The device resistance exhibited a sudden change exactly corresponding to the emergence/vanishing of the CF and a smooth variation corresponding to the outward/inward expansion/contraction of the insulating halo. Anode ablation occurred after repeated cycling, and it is the key factor affecting the endurance of device.