Abstract: Compared the results of laboratory triaxial tests with those of biaxial tests in the particle flow code,the microscopic parameters were determined,and sand characteristics in particle flow code were simulated.Simulation of soil arching by particle flow code was proven feasible through comparison of the results of trapdoor simulated by particle flow code with those of trapdoor tests.Based on the above study,the model of particle flow code was established for simulation on relaxation effect of vertical earth pressure in shield tunneling,and then was used to analyze the earth pressure on the lining,earth displacement and contact force between soil particles considering the factors of ground loss,buried depth and diameter of tunnels and soil properties.It was indicated that the soil arching mainly took place within the range of soil of 1² times the tunnel diameter above the tunnel,and that the earth pressure acting on the tunnel lining decreased greatly because of the soil arching.