Abstract: The tube model is based on the first model and considers the dilution effect of permeating cold seawater.It adds Zone D which represents lower-temperature permeating seawater to the previous model.The model stresses direct mixing of lower-temperature seawater in Zone D with hot fluid in discharge zone,consequently analyses how the hydrothermal fluid is diluted.First,the control equations of two circulations are cstablished,and then it is tried to find numerical solution of the model under different circumstances.Finally,a series of conclusions are found after analyzing the solutions as follows:Permeating cold seawater can hardly invade the black smoker to dilute the hot fluid,while Zone D may become the next discharge zone when the hot fluid discharges at high speed.If there are fissures at the bottom of the black smoker or the shell it has high permeability,evaluations for the temperature,the mass flux,etc.cannot be achieved by considering the vent of the black smoker solely,as the high permeating zone around the smoker may be another bigger discharge zone.Parameters of natural hydrothermal system with obvious low-temperature fluid hardly belong to the parameter space of this model.Different models should be used at different stages of hydrothermal circulation system.