In order to design the best mould surface of combustion gas port in flow control device, based on N-S equation, CFD simulation calculations were made by using Fluent program. By means of simulating fluid movement, the effects of different dilation angle on mixing flow-field were analyzed and theoretical scour of combustion gas port from the gas particle was obtained. The calculation shows that the design of combustion gas port with optimal dilation angle can effectively reduce gas outlet velocity. The result of calculation is elementarily consistent with experimental value.