A mixer is disclosed for use in a reductant dosing system. The mixer may have an impingement floor located within an intended fluid injection path and generally parallel with a flow direction through the mixer. The mixer may also have a first side wall connected along a lengthwise edge of the impingement floor, a second side wall connected along an opposing lengthwise edge of the impingement floor, and a plurality of shelves extending between the first and second side walls. The plurality of shelves each may include a plurality of vanes that promote mixing of an injected fluid. One or more of the plurality of shelves may extend different distances upstream opposite the flow direction.