Lanzhou Metro Line No. 1 is the first underwater tunnel project which crosses the Yellow River and belongs to urban rail transit. The tunnel passes through high permeability sandycobble strata, and the engineering condition is special. The mechanical behavior of segment in different construction stages and the water pressure acting on the segment are analyzed through insitu test and numerical simulation, and the variation rules of segment internal force in different construction stages are revealed. The research results show that: 1) The external water pressure and internal force are significantly affected by the construction of shield in the range of 3-5 rings to the tunnel face, within which the strengthen detection methods should be used at the tensile zone and corresponding measures should be taken to prevent the damage of the segment in the early stage of construction. 2) The water pressure and internal force are gradually unaffected by construction in the range of 6-13 rings and tend to be stable after the 13th ring. 3) The segment designed by beamspring mode is safe, but the construction load should be fully considered.