Abstract: Taking as a research case the shield tunnel of Line 5 of the Fuzhou Metro crossing under the Fuzhou-Xia? men High-speed Railway (under construction), the on-site monitoring data were used to analyze the full process of Crossing construction according to the characteristics of the small-diameter curved shield tunnel in a soft soil stratum crossing under the high-fill subgrade of high-speed rail, so as to research the effect of foundation reinforcement of such projects and the law of construction deformation. The results show that if prestressed concrete pipe piles are used for foundation reinforcement of this project and construction conditions are not available in the metro shield construction at a later stage, the pile-slab structure should be constructed for foundation treatment; after the pileslab structure is used for reinforcement in the soft soil area, the maximum ground settlement is 5.6 mm in the measured data, accounting for 18.67% of the ground settlement control value, which is in the controllable range; after foundation reinforcement is conducted in advance, only slight settlement occurs in different positions of the subgrade in case of undercrossing construction of the shield tunnel, indicating that the reinforcement of pile-slab structure has a very good effect on controlling crossing construction deformation; as the subgrade filling height increases,the soil pressure value of each layer shows an overall increase, and the rate of change in the soil pressure of each layer shows the "bimodal curve" and the soil pressure value in the middle of the subgrade is higher than that close to both sides; before the shield tunnel crosses under the high-speed rail, the change in the axial force of concrete support of the pile-slab structure is roughly split into 4 stages: "linear growth - transition - regrowth - stability", and after the shield tunnel crosses, the axial force of concrete support slightly increases and gradually becomes stable at a later stage. It can be seen from the analysis of monitoring data that the reinforcement effect of pile-slab structure is significant.