Abstract: The vertical loosening earth pressure on a tunnel in sandy soils is affected by the development of loosening zone and arching effect that is influenced by the sand particle flows. Based on the ellipsoid theory of particle flows, the method to determine the loosening zone of tunnels in sandy soils is presented, and the traditional Terzaghi loosening theory is improved. The research results show that, unlike the vertical sliding surface assumed by Terzaghi, the loosening zone of tunnels in sandy soils is an ellipse or part of it, which is related to the eccentricity and loosening factor associated with the shape of sand particles, granular composition, relative density and other characters of stratum. The lateral earth pressure coefficient of sliding surface in sandy soils is less than the value suggested by Terzaghi. Finally, the results of proposed method agree quite well with those reported in other literatures based on the Terzaghi theory. It can be used to analyze the vertical earth pressure in underground tunnel and pipe engineering.