Abstract: Soil lateral stress σ3 or earth pressure at rest P0 was extensively used in geotechnical engineering designs and calculations.The coefficient of earth pressure at rest k0 were tested with a large number of samples buried above the depth of 500 m in areas,such as Hengshui,Huanghua of Hebei province etc.by using various instruments in different laboratories.It was shown that k0 was not always a constant,it was a variable,indicating the deficiency of the formula K0=△σ3/△σ1 in calculating σ3.This formula was basically applicable to calculate σ3 when the vertical effective stress σ1 was less than 200 kPa in shallow soil,but it was defective when σ1 was larger than 200 kPa,and might cause unsafety and accidence when it was used in deep soil.It was found that there was a good power-law relations between σ3 and σ1 for different soils,and different experimental methods might acquire different results of σ3.Therefore,an improved method was presented to calculate σ3,which could provide reliable data for geotechnical engineering construction and research.