In this paper, we use the molecular dynamics simulation to study the phenomena that the granular flow changes from dilute flow to dense flow by increasing the channel width at a fixed orifice of 9.5d (d is the diameter of grains). In the area over the orifice, the density of flow increases and the granular temperatur edecreases with the increase of channel width. The change from dilute flow to dense flow begins at a channel width of 24d, a lower granular temperature( T/m＝0.05 J/kg) and a larger coordination number of 1.2 The mechanism is a structural change in the granular flow, since a wider channel results in more horizontal chains of particles and a lower temperature makes these chains more stable.