Field measurements from practical projects show that the deformation of enclosure structures of narrow foundation pits is relatively smaller than that of super-wide foundation pits; and the latter is often accompanied with kick-out deformation. As the conventional finite element method commonly doesn’t consider the influence of the excavation width on earth pressure on enclosure structures

which causes the design for narrow foundation pit more conservative and results in wasting more

and it also takes more risky for design of a wide pit and creating risks during construction. This paper derives a new calculation method adopting reduction factor of load on the enclosure structure

taking impact of excavation width into consideration. From which the load value is regulated to improve the bar-system-FEM commonly used for foundation pit design. Three centrifuge model tests have been done for foundation pits with different widths of 24 m

32 m and 42 m respectively. The test results show that the calculation results of the traditional bar-system-FEM are obviously incompatible with the test results when the impact of foundation width is considered. The theory of this paper is used to analyze the test results

which proves the reasonableness of the method. This method is a necessary amendment to the bar system FEM and deserves the popularization in the design of foundation pits.