Abstract: The subway crossing project through Hongxu Road is a pipe-roof project with the largest cross-section box culvert in the world now.It is constructed in saturated soft soil stratum in the Beihong Road located in Shanghai.In order to keep the cutting surface stable while the pipe-roof is pushed into the box culvert with many steel grids,it is very important to design rational length of grids with the given cross-section.On the basis of allowable vertical deflection of pipes over the culvert and mechanical model of earth pressure acting on inner plates of a grid,rational approach to design length of grids was given in some practical projects.Finally feasible length of grids was rationally given in detail for the practical project.