Abstract: The formula for rigidity coefficient of soil anchor is given by “Technical specification for retaining and protection of building foundation excavations”(JGJ120-2012). The authors considers that there are two defects in the formula: firstly, the longer length of fixed anchor, the smaller rigidity coefficient of soil anchor, secondly, the rigidity coefficient of soil anchor is dependent only on the material parameters and sizes of bolt and independent of the properties of the surrounding soils. The distribution of frictional resistance along anchorage section is oversimplified in the derivation process of the formula, which cannot truly reflect the load transfer mechanism of anchor. Starting from the cooperation deformation of anchorage section and the surrounding soils, similar to the Winkler foundation model, the action of the surrounding soils against bolt is simplified as a series of shear springs, by introducing the shear stiffness coefficient, a new rigidity coefficient of formula for soil anchor is obtained by establishing force equilibrium equation of anchorage section. Unlike the old one, the new formula shows that the rigidity coefficient of soil anchor increases with the length of anchorage section, but the growth rate is reduced, meanwhile, the rigidity coefficient is closely related to the properties of the surrounding soils.