The computational fluid dynamics method is employed to simulate numerically the characteristics of the sea water flow in the first stage of a multistage flash desalination device.Two applications are presented and compared with each other:one is with two blockboards installed in flash chamber,and the other without.The simulation shows that the largescale circulations and vertices in the water flow govern the flow field in the flash chamber,and greatly affect the flash efficiency.The block boards in the chamber can change the structure of the circulations and vortices, but the arrangement of the block boards used at present is unfavorable to the flash efficiency.