The shield tunnel of Lanzhou metro undercrossing the Yellow River was driven under through high permeability sandy cobble stratum and under high water pressure. A device for external loading test was therefore developed, which introduced a controllable air pressure between the linings and grouting layer, and by regulating the pressure difference, the equivalent water pressure can be then simulated. Using this device together with a tunnel-ground simulation facility, a model test with a geometric similarity ratio of 1∶10 was conducted. The device can reach the point that the water pressure and earth pressure were controlled separately. The mechanical characteristics of segment lining with different external water pressures, soil pressure, coefficient of side earth pressure and segment assembling methods was analyzed. The results suggest that, as external water pressure increased, a significant increasing trend of axial force and a gentle decrease trend of bending moment were observed while eccentricity decreased obviously; as soil pressure increased, a gentle increase of axial force, bending moment and eccentricity was observed; Given a fixed water pressure, increasing effects on the bending moment and decreasing effects on axial force were shown with the increase of soil pressure; Given a normal pressure level, axial force of segment linings increased while bending moment decreased when the coefficient of side earth pressure increased, which made the eccentricity decrease. In addition, the coefficient of side earth pressure shows decreasing effects on the mechanical characteristics of segment linings with the increasing of water pressure; Under staggered assembling, internal force of the segment will produce a mutation at some parts of circumferential and longitudinal joints, besides, internal force of the segment under staggered assembling is larger than that under straight joint assembling.