At present, the horizontal subgrade coefficient of all kinds of soil can not be correctly provided in the engineering investigation report. The m values referenced from existing regulations and guidelines are always in a wide range, which leads to great randomness and variability for the selection of m value in design. On the basis of hypothesis and simplified analysis, the formula for calculating the equivalent m value of foundation pit soil layer is provided, which considers soil gravity, excavation depth, lateral earth pressure coefficient and shear strength. By using the measured horizontal displacement monitoring data, the nonparametric regression method is used to back analyze the coefficient Δ1 and Δ2 to correct the formula of the equivalent m value. Taking a shallow highway tunnel in Kunming for example, the back analysis method is analyzed and verified. The results show that the method can not only reduce the m value range given by the regulations or guidelines and improve the calculation precision, but also can constantly modify the equivalent m value of the foundation pit soil according to the depth of the foundation pit, and then realize the dynamic control of the foundation pit.