When the cast-in-situ cut & cover tube section at the west island of HZMB is joined with the prefabricated immersed tube in the sea floor,part of steel cylinder around the joint place must be dismantled.However,the west island is required to be satisfactory for the dry construction.Therefore secondary seep-proof screen shall be set inward the island.3-D CAD is used for the spatial simulation of the whole construction process of secondary seep-proof screen,by which we can probe into the weak points and optimize the design of the seep-proof screen.In this way,the seep-proof surface is ensured to be continuous,and the reliability of seep-proof screen is improved.A 3-D finite element software LUSAS is also used to analyze the stress and displacement of the secondary seep-proof screen.The data gained from the software match well with the measured values on site.