Abstract: In measuring free-field in-situ stresses, the passive arching effect is aroused over the cylinder surface of earth pressure cells because the stiffness of the pressure cell shell is much greater than that of the surrounding soil. Partial soil weight is transformed to the pressure cells owing to the passive soil arching, resulting in larger measuring values of earth pressure. According to the soil arching theory, an equation for calculating the measuring error induced by the passive soil arching is deduced. Compared with the existing formulas, the new equation includes soil friction angle and cohesion rather than the elastic modulus, and reveals the interaction between the soil and the pressure cell. It is helpful to increase the compactness of the soil surrounding the pressure cells, and to decrease the density of the soil upon the pressure cells for weakening the passive soil arching effect. Moreover, enlarging the diameter ratio of pressure cells to cell diaphragm can diminish the influence of the passive soil arching.