Aiming at the argument of vacuum transmission in drain board under vacuum preloading,this paper analyzes the concept of vacuum and negative pore water pressure under the theory of fluid mechanics,and points out that the real reason of soft soil drainage and consolidation is the decrease of pore water pressure in the drain board.The total potential of pore water in the key position of drain board is calculated by the theory of fluid mechanics.Then the action mechanism and flow process of pore water pressure is analyzed based on the lowest energy principle.So the changing rules of pore water pressure in the drain board are obtained.The results show that:1) the changing rules of pore water pressure in the drain board relates closely to the ground water level,vacuum degree under membrane and drainage board deformation;2) in the drain board,the simplified distribution pattern of pore water pressure reduction is a broken line,and the line’s turning point is located in the ground water level;3) the pore water pressure reduction changes a lot above the ground water level,and the attenuation rate is 10 kPam.But the pore water pressure reduction remains unchanged below the ground water level;4) the more deformation of drainage board occurs,the more resistance of drainage is.When the drain board breaks,the vacuum load under membrane can not transmit to the deep ground.