A knife sharpener comprises a base, a motor, a grinding wheel assembly, a rotary shaft and a spring. The working channel between the edge of the grinding disc and the grinding awl is automatically adjustable to accommodate different blade of different thickness. When the knife sharpener performs grinding operation, the grinding wheel assembly can simultaneously grind both cutting sides of the knife. In addition, the knife can be clamped firmly through the three contacting points of the grinding disc, the grinding awl, and the cutting edge, so that the operation of knife grinding can be successfully performed.