Abstract: CdZnTe single crystal growth process with detached vertical Bridgman method by FLUENT was simulated. Two kinds of boundary condition of the upper melt were calculated: (1)non-slip wall;(2) free surface. The gap width (e) between the crucible wall and crystal and gravity were the two key factors for the crystal growth process. The temperature distribution, stream function distribution and the solidification interface shape were displayed and analyzed both in the microgravity and the gravity condition with e equals to 0 mm, 0.5 mm and 1 mm, respectively. The simulation results showed that in the microgravity condition, when the upper of melt is non-slip wall, the flow strength of thermocapillary convection increases with the wider gap width. However, when the upper of melt is free surface, the flow strength shows the opposite trend with the increase of the gap width. In the gravity condition, the flow strength decreases with the increase of the gap width, which will improve the crystal quality.