A fine threedimensional finite element model of a segment ring lining structure is established by means of numerical simulation and on the basis of construction of Zhanjiang Bay seacrossing shield tunnel. The deformation of a segment ring and the segment joint are studied under the effect of different water heads. The modified routine method is adopted to analyze the stress distribution of segment structure under the maximum water head. The circumferential reinforcement stresses of inner and outer segment are calculated and compared with monitoring data. The results show that: 1) Under high water pressure, the single segment ring is deformed into “horizontal ellipse” shape; and the segment structure deformation and joint opening have a linear relation with variations of waterpressure. 2) The absolute joint opening increases by 0.5 mm when the water head increases by 10 m. 3) The reinforcement stress calculated by means of modified routine method is consistent with the monitoring data, so as to reflect the actual force situation of tunnel segment quite well.