Abstract: Up to now, the subway crossing project through Hongxu Road is a pipe-roof project with the biggest cross-section of box culvert in the world. It is constructed in saturated soft soil stratum under the Beihong Road located in the middle ring-road of Shanghai. During pushing the box culvert in the front of which many steel grids are inverted, earth pressure on inner plates of a grid will vary with excavation of soil near the back end of the grid. Its general rule is that earth pressure is of different value at various distance from cutting surface. According to the mechanical equilibrium conditions, earth pressures on inner top and bottom plate are exponentially distribution along the longitudinal direction of a grid and those on inner side plates are superposition of exponential distribution along the longitudinal direction and linear distribution along the vertical direction of the same grid until reaching their corresponding original status at the fore end of the grid after attaining new mechanical equilibrium status. It is the important basis of rational design of grid sizes. In the end, the theoretical calculation method is proved by the model test. The distribution mode is given in detail by the practical project.