An investigation of the formation mechanisms of channels milled by abrasive waterjet (AWJ) on an amorphous glass is presented, based on the jet flow characteristics and erosion theories. It is found that the channels are formed through four different zones, i.e., an opening zone, a steady-cutting zone, an unsteady-cutting zone, and a finishing zone. These zones are respectively associated with a secondary viscous flow generated upon the jet impact on the top surface of material, a turbulent flow developed during the penetration of the jet into the material, a transition or laminar flow at the downstream of the jet, and a vortex and damping flow caused by the accumulation of the low-energy solid particles at the bottom of the channel. Bulges are found at the channel bottom and close to the channel wall machined at high nozzle speed as a result of a force induced by the acceleration/deceleration of the moving nozzle when changing direction during the operation. It is found that large high nozzle traverse speed and/or small standoff distance result in lay marks featuring with parallel grooves at constant distance. Sawtooth waves are observed on the machined surface when the cross feed is small. The effect of process parameters on the channel formation process as well as the milling depth, material removal rate (MRR), and wall inclination angle are then discussed. It is found that the milling depth and machined surface quality can be reasonably controlled through the selection of process parameters.