Abstract: Objective: Choosing pile slab structure as the subgrade form for tramline will significantly increase the construction cost. In contrast, the cost of the composite pile\|plank structure foundation is lower, and it can fully utilize the synergistic force between the pile, slab, and soil. Therefore, it is necessary to conduct an in\|depth study on the mechanical characteristics of the composite pile\|plank structure subgrade for tramline. Method: Based on the tram demonstration line project in Shanghai Songjiang, the stress and deformation characteristics of the composite pile\|plank structure subgrade in deeply soft soil areas are elaborated. By using numerical simulation methods, the stress and deformation characteristics of the soil, pile body, and pile\|soil under the subgrade middle plank are analyzed in detail during operation and construction periods of the project. Furthermore, the mechanical properties of the composite pile\|plank structure subgrade under different pile lengths are analyzed. Result &Conclusion: The additional stress on the soil under the mid span slab shows a pattern of first decreasing, then increasing, and finally decreasing again with increasing depth, reaching a peak value at a depth of 1-2 m below the pile bottom; the bearing capacity of the pile is mainly provided by the lateral friction resistance of the pile, and the differential settlement value between the lateral soil and the soil under the mid span slab is positively correlated with the lateral friction resistance; with the increase of load, the lateral friction resistance gradually reaches the limit value from bottom to top; as the pile length increases, the settlement of the soil under the plate gradually decreases, keeping within the range of the ultimate bearing capacity of the pile. Appropriately reducing the length of the pile will not significantly reduce the size of the upper load that the pile can bear.